I was bored by a lot of things we did in Cub Scouts, and all the cool kids dropped out when it was time for Boy Scouts so it was just me and I only lasted 6 months or so. Pinewood derby cars were fun, but I was also inordinately jealous of the kids whose parents could afford the sticker kits or graphite for the wheels. I remember camping nights being fun and also that the one Regalia we attended was amazing for how many people were there, but the one thing we did during Cub Scouts that always sticks out in my mind as amazing was the time we built a pykrete canoe.
Our canoe was a lot thicker-walled and simpler in construction than the one the BBC made above (I recall ours looking like a dugout canoe from a huge tree, with walls over a foot thick and not being nearly as deep in sitting area). According to my brief look at the internets, theirs tipped and melted pretty early.
I don't even recall it being called pykrete at the time, but we clearly built a functioning canoe out of 14% sawdust and 86% ice: pykrete. We built the thing one winter day and tried it out in the open water below the lock and dam in our town, and though it manuevered like a pig it worked marvelously. It lasted for what I remember as over an hour on the Mississippi until we got bored of sitting in it and doing not much else because it was so hard to paddle. When we were done, we just left it in the water to disintegrate. It was gone the next day, presumably after melting.
Pykrete is a weird substance. It floats like ice, but it does not shatter or crack. It's repairable with simple saltwater patching. It has the same basic principles and strength as concrete, though of course it can melt, especially in fresh water as saltwater tends to have some insulating properties. And now the weirdest part about pykrete: its ability to float and be easily/cheaply manufactured led to British authorities doing full-scale testing in WWII for the feasibility of replacing expensive steel aircraft carriers with giant floating pykrete islands. They test-built some smaller pykrete "watercraft" in Canada in 1942. The islands floated well and lasted for months when at that size (only the edges melting slowly), but the ends sagged under their own weight and plans for trying larger pykrete islands were scrapped.
Happy Hamsterdam, all!