Question Islands, bridges, obsticals, boat and zombies oh my!

Hi All.

I've implemented A* and Contextual Steering, but looking for ideas for Chasing a player when they are on the island and when they are in a boat off the island. I'd like something 'inexpensive' the would be hydrophobic (not walk on water- hahah!) but still try to get as close to the player as possible.

Also It would need to incorporate, no merging/smushing (occupying the same space or overlapping of other attackers but maybe climb on top of them! Piling would be super cool!
Yes World War Z/I Am Legend/Resident Evil style! would be super-cool!

I've looked at a bunch of ideas but not sure which might serve me best.
Any ideas are welcome. I can barely get 20-30 zombies chasing the player before my machine chugs, thus the need for a performant / super efficient /low bandwidth solution.

Any help/ point me in the right direction welcome!
I'd prefer to roll my own vs. off the shelf / Unity store / navmesh solutions.

Thanks!

ps, also I trying to have procedural terrain so need something that will take that into account too. Wow! that's a big list of requirement, sorry, but y'all have bigger brains than mine so I have faith!


Ps. Actually is there someplace i.e Not NavMesh that I should post this??