You may consider the following depending on the layout of your boat: a standpipe.
A standpipe is a straight metal pipe either welded or screwed onto a thru-hull base. It can be secured by bracing if needed. It extends above the waterline and is capped with a gasketed (again, usually metal) cap. It resembles the old style oil
fill tubes on the sides of North American houses from when oil
was a common furnace fuel
Down the pipe, below the waterline, are welded-on T-fitting threaded to take sea cocks. Put in as many as you need and can safety
fit in regards to access.
The advantages are many in that you have only one "intake" hole in the boat. Fitted with a hinged screen
or filter, you can, should you suck in fish
and/or debris, open the top cap, see what's down there, and use a length of dowelling to clear the entire standpipe.
An alternative, "belt and suspenders" approach would be to have a single
, large seacock near the centerline/midpoint of the boat (to ensure it is always below the waterline). Then, you tap an appropriate pipe with the required T-fitting and smaller seacocks for raw water
intake, A/C, seawater domestic supply, and head
, and fit it to the seacock. Then, if a T-fitting fails, you can shut off the main seacock and fix it (or reroute its hose to a spare fitting) while underway.
There is no particular reason that the seawater intake must be forward of the engine, other than that forward is usually deeper and less likely to be above the waterline or in part-spray, part-air if heeled. Getting "gulps" of air in the raw-water intake circuit could indeed lead to problems, so you have to figure out the angles. Generally, making the lower-most T-fitting on a well-placed standpipe would do the trick.
As for positioning, you can "hide" in under saloon
stairs or inside a piece of galley
cabinetry, just as long as you can reach all those seacocks/shut-off valves and can look down the standpipe to see blockages and deal with them.
The standpipe in my steel motorsailer
saved my engine when I noticed the engine temperature rising and noted that the pump was running properly. I looked down the standpipe and saw that we had sucked in a large, thin plastic bag. I was able to push it down and out and we resumed motoring with no further incident.