25' of clear vinyl tubing, and a bucket of water, colored with food coloring, or as in this case, with raspberry jell-o because the grocery store was having a deal. $10.37, batteries included.
To get the fluid into the tubing, you get the tubing into the fluid. Anyone who's ever siphon-cleaned an aquarium knows the deal. Feed the tubing into the water a bit at a time, keeping the one end below the surface the whole time. Hold it down with a rock or something.
Cool little tip: 1/4" tubing is the perfect size because it can be plugged with an ordinary pencil. Plug one end of the tubing to prevent the fluid from running out when removed from the bucket.
So the concept is dead simple. With the ends of the tubing unplugged, the water in each end will always and forever be level. Tape one end to each side of the boat.
And futz with the boatstands until things line up....
Presto! level boat.
Oh, but it's best to level it fore-and-aft before leveling side-to-side. As many times as I've done this, I always seem to forget that little gem.