OK. 30' boat, so one head
, one shower
, one water heater, one tank, and one pump. A 2 or 3 gallon per minute automatic pressure pump, such as a Jabsco
Consider adding a sight gauge or some other way to tell how much water you have - I do it by making the fill pipe clear vinyl, so I can see the water level in the tank. Simple, it works.
Consider piping the whole thing in PEX or CPVC. PEX is more flexible, CPVC is cheaper. Avoid rubber hose.
A six gallon hot water heater will cover the needs of a couple for daily showers as long as no one goes Hollywood.
That's really all you need. You've got a simple system. I added recirculation of hot water so we don't run the water to get to the hot, but you have short runs and if you use 1/2 inch pipe you won't lose much.
You'll be able to detect a leak by the pump cycling as pressure drops. You get a better read on that if you don't have an accumulator, because there's no stored pressure other than the very slight bulging of the CPVC.
I don't think that a flow meter is going to add enough information to be worthwhile. You'll know fast versus slow flow from the sound of the pump, and total use by looking at the sight gauge. Calibrate it by filling the tank from a five gallon can if you want to know more.
Good luck with it. Running the CPVC or PEX through an old boat may teach you why sailors are known for their salty language.