I have never seen an octopus, but jellyfish, uug. We got stuck in the middle of a swarm of small box jellies in Trinidad so big we had to shut every thru hull
on the boat. It took hours to pull them out of everything, and bits of them from all the plumbing
A strainer (internal or external) needs to have a thru volume of x gallons/min @ Y head to feed whatever system it is plumbed to. Engines, generators, AC pumps, all have specced thru hull
sizes based on the engines cooling
demands. When you add a strainer (either type) you reduce the cooling water
getting thru the system by some amount. That restriction amount is based upon the surface area of the strainer minus the surface area of the grid acting to keep stuff out.
A strainer basked has massively more surface area than an external strainer, so even tho they filter the same size particles the internal has a lot more water
pass thru. As I mentioned it doesn't matter much for heads since water thru rate isn't critical, but for systems that need cooling it can be.