As one who has done it, they float well. How long I dont know, as I made it a point to get it to shore asap. There is a lot of air trapped in the wings, I expect it would be a long time before it sank.

In my case, I was flying again in under 24hrs, the only electronics I lost was the one thing that was supposed to be waterproof, my ELT. Aircraft damage was limited to one cracked gear door.

I had heard a briefing on landing in the water, vents shut, gear down, away from shore(dont know what is hidden under the water), wings level, slow as posable, and flaps, spoilers closed.

The gear down part is the counter intuitive part, but this little kitchen demonstration will show why. Hold a spoon so it is just dangling from the handle between you fingertips. Try to hold it so the spoon is down, but can pivot in the convex direction. Turn on the tap and just touch the bottom of the spoon to the stream. It will suck into the stream. Gear down will help avoid this.

Also most gliders have the gear well sealed so the water will not enter from there.

In my case in the cocpit I had about 1.5 inches of freeboard at the cocpit sides when I was in it, but watter was comming in from the nose hook, and comming in the fusalage from the rudder. I got out and side stroked and towed the glider to shore. It towed very easily because it was such a clean shape.

As for safety, if the water was not dangerously cold, you are probably safe in the water, but you risk damaging you electronics. Note, in the 99% category, you dont often have a chance of drowning in a land based Landout.

These days, in my ash-31 I would be very reluctant, but in a composite "gravity glider" it is way better than the trees...