To sign boat, cup your hands together in a boat shape. Then move your hands up and down while extending them forward. The sign should look like a little boat bobbing along in some gentle waves.
This is quite difficult to sign precisely, so you will often see babies simplify that by pushing their hands together and moving them forward.
We use boat with a little toy boat that we like to play with in the bath. It is a nice little sign, because the boat bobs along much like the boat sign, helping to cement the connection for your baby.
Depending on your proximity to navigable waters, you may be able to teach this sign contextually. When no real boats are close by, consider buying your baby a boat toy to use during bath time.