Every one of your sinks and heads are connected to thru-hulls below the water line. Do you close those every time you leave the boat?
Even with the raw-water strainer (the Groco is the best) above the water line, once the air is removed you will have the potential of a siphon effect if there is any breach of the raw water lines below the water-line. So long as there is no breach, and the discharge line is above the water line, you have no issue. You will allso have no issue if you simply close the through-hull when you are not aboard the boat.
Your selection of the March Pump is a good one as these are air cooled units and are less prone to failure as are Cal
Pumps. One thing to keep in mind as you install the pump is to ensure that the two lubricating holes at either end of the pump motor
case are oriented upward. (If necessary you can ease the mounting brackets on the pump and rotate the motor
until the lubricating holes are up-ward when the pump is in the mounted position. The pump assembly on the end of the motor itself can be re-oriented without difficulty to suit the discharge direction). The pump bearings need be lubricated roughly every 3 to 6 months, depending upon use, by injecting some 3-in-1 oil
(ie 20 weight lube oil) into the lubricating holes. As these can be hard to spot in the awkward--dark--mounting positions that the pump is likely to be in, put a little white paint
around each hole--use a Q-tip to keep paint
out of the hole itself--before you bolt the pump in place. With that it's not so difficult to see where to inject a squit of oil
. (I actually keep a little bottle of 3-in-1 oil in a hook next to the pump to ensure its always there.)