Is it possible to add tugboat spawns to a map? Anything I should know?

Should be, just need to place the spawn points. You can't really place spawn points directly on water anymore as of the Rust water update, as the raycast won't hit, but you can move a spawn point after placing it somewhere else.

First attempt resulted in like 3 tugboats accumulating on the one spawn point, even though the max was set to 1 lol. Turned on some more spawn group/point flags to see if it will help.

I'm also wondering if it will keep spawning more if people drive them away, or if overall server limits will still apply.

Yes, driving them away allow another to spawn. Probably the boat drifts enough in the water that it's considered sufficiently away from the spawn point to detach.

We can possibly add an option to not detach in a future update.