Sea towers would be a feature completely revolving around naval warfare, something which in general is rarely present in the game's sieges otherwise. It would create an economy and demand for ships. Regarding the travelling part, I don't think it's a big deal for people. Ample time to prepare if they want to participate. Then again, it gives a different value to distant holdings nobody else wants otherwise. It's a fun event, imo the most fun I usually had in DFO.
However, you have to take into account why we are introducing local banking.In New Dawn, its main purpose is to be a convenience feature.