There are quite a few posts about this. The mass of your boat is probably not heavy enough to be stable when a player jumps on the boat. The reason it was tipping is because the weight of the boat is sitting above the part that’s floating it which makes it unstable.
A simple trick is to make the boat Density very light, and add a high Density Part below the water just like the counterweighted keel of a sailboat. That Part can be CanCollide false and transparent so it doesn’t interfere with gameplay.
It takes some tweaking of the low boat Density (less than 1, the Density of Water) keeping the boat floating and the keel Density and size. If the boat still tips make the keel larger or more dense, and if the boat starts to sink then decrease the boat parts Density to help it float.
You want the boat model Mass to be high enough that player physics don’t affect it, but light enough to still float.