There's another thread running about this as we were trying to decide which to stock, and also I'm doing an article for my NCT newsletter about swim nappies. You can find the thread here.
We went for the Happy Nappy which is made from thin neoprene (like a wetsuit), so very grippy (wet babies are slippy!) and helps to keep heat in. They are supposed to be excellent because they stop even breastfed poo from getting into the water, which nets just don't do.
The disadvantage is that you can't open them like a nappy, so if there is a poo situation you need to pull them down which is trickier to deal with. However you can use a paper liner inside which helps.
You can use a disposable nappy inside them - MissGolightly suggested if they're just wet you can dry them and re-use them. This has the advantage that you're collecting more wee before it goes in the water, but you have the poo protection from the Happy Nappy.
That's what we're doing, but in the immortal words of the BBC "other nappy brands are available"
Check out the other thread and I'm sure you'll get more responses from other people!
we go to waterbabies, they have a shop online, and they insist on happy nappy and a swim nappy, seems to contain everything really well. Just a note of advice, proper swimming pools are really cold for los,,,check your pool temp, as you may need to buy a baby swim suit, my lo was in the learning pool for 5 mins and was navy blue, we did complain about temp just to be given some guff about chemicals etc, but they did confess to have lowered the temp in the pool
ps i dont work for waterbabies,,
The only poo my LO has done was in teh dispossable and it held it in fine (was breast fed poo too).
Personally i dont see the point of using a disposable and a reuseable together (especially as the re-useables are expensive)...surely you go for one system or the other???
Would agree that pool can sometimes be chilly, my local pool has a couple of sessions per week for disabled people where they warm the pool up so if i then go to a public session after this it is nice and warm.
Also at 4 months you may only end up being in water for 15-20 mins before they get chilly or fed up.
The other thing i did was went along to one of the private baby swimming sessions for a free taster, this showed me the holds to use and some ideas for what to do. The lessons were £8.00 per half hr which i personally didnt think was worth it so after using free session to bulid my confidence in what to do then just went to local public pool with my friend.
Using both a disposable and Happy Nappy means you've got an iron clad baby's bottom however the Happy Nappy on its own is just fine and works brilliantly. It also helps keep baby warmer and less slippery.
Many swim schools are insisting on them now with more in the pipeline.
I like the konfidence.com double nappy system. its a lot like the happy nappy but made in cornwall. So boy wears a washable swim nappy that is very adjustable and konfidence.com claim fits 3month-3 years and a nappicover made of neoprene ( like the happy nappy). No pooh can escape and any he does is in the inner nappy which unpoppers flat to make clean up easy.
Our local pool is 30-31 degrees and i use a babywarma wetsuit ( also konfidence.com). even with that on him I have to watch he doesn't get too cold and if he does get him out.
Wet suit does make it lot easier to hold him when wet.