When you kill someone

[TeckXKnight]

i died by being rammed into with a boat D:, that or they unarm striked my boat till it broke then unarm striked me to death while i drowned, not sure

They destroyed your boat and drowned you.

[Girlinhat]

You can perform actions while on a boat (or pulling a cart!) and that includes destroying. So they boated over to you, then destroyed your boat as normal.

[MagicManICT]

i died by being rammed into with a boat D:, that or they unarm striked my boat till it broke then unarm striked me to death while i drowned, not sure

They destroyed the boat as you would destroy anything else. Since the boat isn't on a claim, you really don't leave vandalism scents. (Or murder scents for that matter.) Just another way to grief other players.