I am not entirely new to boats. I've been on several cross-Channel ferries; I nearly killed myself in a canoe on the Ardèche; I've spent time on a Turkish gulet (blissful); I've done feluccas in Egypt; and I had a week on a barge on the Welsh border. I have discovered I actually rather like boats: I like the portholes, compact kitchens and cooking on the move. In fact I like boats so much I'm seriously considering buying one and making a new life on the water.

So a recent trip to Portugal to spend a few days on a houseboat on an enormous reservoir in the middle of nowhere was a perfect opportunity to test my "sea" legs. Even better, this boat came with sunshine and a vast swimming pool, and without locks, tunnels or horizontal rain (though I like all that too). The trickiest element, apart from the chemical toilet (of which more later), was parking the thing, but then I find that difficult in a Nissan Micra.

As my children grow older there are many joys, but one of the small sadnesses is that they are not quite so easily pleased. Whether it's packed lunches, T-shirts, birthday parties or holidays, they now have their own strongly held views. There was a time when they were thrilled to spend a week with us anywhere – in a longhouse in Wales, on a canal boat in Shropshire, at my parents' place in the Midlands. Now holidays have to be carefully negotiated around friends, girlfriends, sporting events and parties. At home we rarely spend all day together, just the four of us. And while I like boats, do they? How would we survive on a cramped motorboat on a deserted reservoir?

But that was before I'd seen the reservoir. Alqueva lake, in the unspoilt Alentejo region of southern Portugal, is the largest lake in the country, covering a surface area of 250 sq km, 83km from end to end. It was created in 2002 after the majestic Alqueva dam was built on the Guadiana river to irrigate the impoverished and arid Alentejo. Locals were promised much from its construction in 2002. Something may still materialise, but probably not enough, and not quickly enough for the poorest part of a country in an economic mess.

The Alentejo covers a third of the country and is its least populated region. As you pass through, by car or on water, what strikes you is its emptiness – it's a vast, barren, unchanged and unchanging landscape. There is little development, just a few isolated villages and stunted trees.

For tourists, that must be one of its great pleasures. The only tourists we met were an English couple who had been on a boat for two weeks. He was a keen fisherman, crates of Super Bock piled up next to the rods. I have no idea how she kept herself entertained: maybe she was a keen open-water swimmer.

But for a few days, with a couple of kids, it was the most brilliant fun. The boat (worth €150,000) was a mobile playground. We all wanted to be captain and fought to be at the wheel. We all loved the GPS, and the sonar system, which showed depth of water and obstacles, including large shoals of fish. And it was lovely to be out on the water, self-contained and independent.

We motored gently down the length of the lake (travelling at about eight knots, which feels very slow when the sun is setting and you're struggling to reach the next jetty to moor for the night, but super fast if you're swimming and your family think it's funny to drive off without you). Really though, these boats are incredibly sedate – no driving licence or sailing experience is needed – and virtually without risk.

The nearest we came to disaster was on our first day when we radically underestimated the time it would take to reach our destination. The light began to fail and there was no coaxing this boat to go any faster. I don't think you're meant to travel at night and the advice is not to drop anchor in the middle of the lake because there's a flooded village and lord knows what else down below. (Of course we made it to the next jetty and then had a spectacular row as we tried to park.)

Mapped from above we probably steered a peculiar course, but it was fun. If the youngest was driving, we went into 360-degree spins just for the hell of it; the older boy took it much more seriously, while all the time listening to Portuguese radio at full blast. We often stopped to hurl ourselves into the water, which was clean and warm. And we all loved the kayaks which we pulled behind us, often with the boys in them, being towed like mini banana boats.

We could have taken bikes, which might have been useful for exploring – villages that long predate the lake are quite a way from its shores. We took fishing rods, but didn't catch a thing, despite one night seeing hundreds of fish surfacing in a seething mass of plops and splashes. There is sailing and water-skiing from the Amieira Marina (our youngest had a go at wakeboarding), and there are visits to nearby villages. Marina staff will help you with taxis and activities – they'll even do your shopping for you, and are very, very nice.

We didn't do culture or villages. We did nice food – at the restaurant at Amieira Marina and at a fantastic former olive oil factory called Sem Fim on the outskirts of fortified Monsaraz, where the food was delicious, wholesome and hearty, and the setting was even better. We also spent a day in Evora, one of Portugal's best preserved medieval towns, with its Aqueduto da Água de Prata (aqueduct of silver water) and its Capela dos Ossos, a macabre chapel built by 17th century Franciscan monks entirely out of bones and skulls as a memento mori or reminder of death.

Traditionally Brits don't make it to the Alentejo. It's the Algarve for us. But a new flight in and out of Beja airport (which is very hard to find – if you're in Portugal, I mean. I'm not sure how it works from the air) has opened the area up and I hope it does well. It is widely described as a hidden gem, and it really does feel like that.

Ah, the toilet. Yes, the boat comes with all mod cons. TV, shower, fridge, barbecue … and chemical loo. I didn't quite conquer that one. You turn the handle this way and that – water comes, water goes, things disappear and then they reappear. I just didn't get it. I am famously squeamish about this sort of thing, so it is some measure of the joyousness of this holiday that even the primitive loo could not put me off.

• Sunvil Discovery (020-8758 4722, sunvil.co.uk) has weekly flights from Heathrow to Beja from £169 return. Amieira Marina (amieiramarina.com) has boats sleeping from two to 12, with the four-berth Duo from €163 a night, plus a one-off tax of €76. For more information on the region, see visitalentejo.pt, and on Portugal, see visitportugal.com