Prob'ly due to...
the higher head from the faucet to the shower head. The onboard water pressure is that high to beging with...add 5' of hose that goes straight up, and you're gonna lose some pressure. If you have an on/off "switch" on the shower head, you can see what I mean...leave it off for a few minutes...the pressure builds up in the hose from the shower head...dies as the pressure is relieved when you run the shower.On a boat, this is usually not considered a bad thing. The water supply is limited, and the idea is to save as much for drinking as possible, using as little for everything else as possible. In fact, someone once wrote that if you want to experience what it's like to take a shower on a sailboat, stand in a very small closet for 5 minutes with a bar of soap and very wet dog.