On most boats a dedicated through-hull is indeed the way to go, but on my boat I have an older solution...a standpipe with multiple T-fittings. Basically, it's a three or four inch pipe open to the ocean that rises to a point several inches above the waterline and has a pipe cap that screws on and is frequently gasketed. From this are run several 1/2" or 3/4" fittings with their own seacocks which feed the head, the engine, the A/C. the watermaker, etc.
Standpipes used to be also used in galleys for a drain you simply plugged at the sink...no seacock, no need to turn it on or off.
The advantages of having only one intake hole in the bottom are obvious, while the disadvantages include not many modern boats having the vertical height in an accessible position to install this sort of set-up. Also, being able to literally remove the cap and peer down the pipe makes it easier to diagnose clogs (including fish needing removal), but if a seacock fails, you would do well to have a big wooden plug you can jam into the bottom of the boat in order to change out the faulty seacock.
