You need a material that is lighter than water (in scrap mechanic basically everything exept metal). You then need to place a water plane which says how high the water is in the world. Build your boat mostly out of that lighter than water material then place a water activator on it (the purple one is the best accuracy wise). After that your "boat" should float if it is on the water. if it flips over then you need to add some metal to the bottom of the boat to act as a keel, and if it bounces alot I suggest doing this as well. If the bouncing continues add some of the green drag blocks. They spread the drag across all of them that are placed, and so if you put one in each corner it should help signifigantly. The new water activator (purple one) already has drag integrated however so the drag blocks should not be needed. To add propulsion to you boat add a prop on a bearing (I reccommend adding two: the mirror version on the other side) rotating these propellors under water will produce a force. That should be all, cheers and have fun!