White foam CAN absorb a small amount of water. This can happen if you leave it submerged for an extended period of time. Don't do that. Take your airplane out of the water when you are done flying, and store it out of the water. However, it has never been documented where white foam absorbed so much water it would not float.

Built-up floats CAN absorb a LARGE amount of water. This can happen if you hit a stick on takeoff or landing. If you hit the stick just right, your float can be full of water in 0.00000263 seconds and you will wonder why your airplane mysteriously just disappeared. Or, if you actually take off, the plane will swap ends as soon as the water runs to the rear of the floats. This will lead to a curtailment of your day's flying activities and an airplane-sized debit against your Visa Card, if you're lucky. You COULD lose your engine and radio too. Built-up floats can also absorb a teaspoon of water, which is enough to feed massive amounts of ROT. If the balsa rots off of foam floats, they still work. If the wood rots in built-up floats, you get to hire a diver to look for your motor.

White foam floats are NOT fragile. I have an old set of white foam floats that have been on at least 5 planes. 2 big Lazy Bees, a Stik, a FunFly, and a SPAD. SOme of the planes have had the floats on and off several times. They are balsa sheeted and covered with film, They have been patched and fixed and are tattered, but still serviceable. However, I could break one in half and the pieces would still float, and I could glue the pieces back together and be flying the next weekend.

With foam floats, I boneheaded my plane into granite riprap and holed the balsa bottom. Went out, grabbed the plane, dried the floats, slapped on some packing tape, and kept flying, as has been documented in threads in this forum.

Pink, blue, and gray foam are OK for floats also, just heavier. The foam is only the filler and it is lightly loaded. The structural part of foam floats is the sheeting. Even though I have the old set of foamies only covered with Balsa and film, now I seal the balsa with one layer of 3/4 glass cloth. The cloth adds a lot of strength, the resin(or polyurethane varnish-easier to work with) seals the balsa. Good (Snell approved) crash helmets have a fiberglass shell and a styrofoam liner...think about it.

I have only been float flying for about 10 years, and r/c flying for about 20. Some of this stuff I learned by doing it the wrong way first. Generally now I can go flying and have a good time and come home with the same number of airplane pieces and credit card balance that I took to the lake. I call that a success.