Heat a bunch of gallons of water to just below boiling then poor it in the boat. Suspend the boat in the air with rope etc and let heat and gravity work together. Be very careful using any sort of probing items (broomstick, 2x4's etc) you could easily push a hole right through the plastic when it gets soft.
Try gravity 1st then if all else fails maybe a broomstick,but only if the boat is unusable due to the dent. otherwise just boat with a dent.
" ...this is the hardest sport I've ever tried!!",E. Brown, 2003