In April you never quite know what to expect but on average it should be quite hot during the day and coolish at night. You might get no rain or you might get a downpour, or anything in between. For some average stats have a look here. You can see weather history on http://www.wunderground.com but I can't post the link because it keeps crashing my browser! It will be warm enough to go to the water parks. The sea is never very warm even in high Summer but in April you will see some Brits swimming - but NO Portuguese! Bear in mind that the sea can be quite rough in the Algarve and April might be a bit early for water sports to be available. Perhaps someone else can help on this one.

All the bars and restaurants will be open from Easter (first weekend in April), and if you're there on 25th April there be a great atmosphere in the Old Town and a fantastic fireworks display as they celebrate Freedom Day.

I went late March a few years back on a £9pn 4* hotel deal off Hotelopia so obviously earlier than when you are looking and most bars and restaurants were open. I would hope that another month on the weather will have improved for you as it was pretty grim when we were there and any form of sunbathing or swimming was well out of the question.
We did head to Lisbon in the car for a day trip and that was the only day it was sunny and warm. It was still enjoyable though, and of course the saying of "it never rains in pubs" seemed very apt for that time of the year.

Yes, I'm afraid it's a bit like asking what the weather is like in England in April. We had a very cold week in early May a few years back but it had apparently been a heatwave just before we arrived. And we sunbathed in our cozzies at Arrifes in January with our 3-month old granddaughter a couple of years later - and went in the sea!

But "on average" it should be better than in the UK.

Slightly but weather related and a good example of how anything can happen was our experience of Corfu in August a few years back. Arrived in glorious 30c+ temperatures and for the next two days it was lovely, then it rained solidly for three days!!! And I mean solidly...the only thing we could do was find a bar, grab some papers and enjoy a few drinks. We were only there for a week as well........

Funnily enough the advice I was given for the Algarve in late March was "it should be better than the UK" and that was good advice, because it was better but I think that was more to do with the awful UK weather as opposed to any great shakes in Portugal. April should be better still......and anyway, a holiday is a holiday after all.

The Algarve water parks are not open in April apart from Zoomarine's indoor sections. It's too cold!

In have been here in April when it has been 88F in English money and equally when it has been 45F. You never can tell as the weather is often unsettled in the spring.