I think these canoes should be quite long. By the look of it, compared to the chair, I'd say about 6-7 meters. Not too large I'd say. But if you color the front one darker it'll appear smaller. The trees are very convincing, I have to make something like that as well. I do have some very LP trees consisting of 8 random planes with some leaves, but a sphere might work better.
And yes, crap is hard. You can't model too much crap at once, or it'll float. Procedural/internal crap is better. Maybe some tiny stones (pop) with irregular and rough displacement may do the trick. I did a population of fallen trees once with fake stones.
