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.
Minaret,
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.