Is this "normal" or unusual ?

In my cruisers cabin, the dc system consists of 1 light, pump for
demand water, and cd player. There is a separate 12 ga line back to
the battery for the cd player (installed by previous owner) and the
light and pump are wired I assume as original.

I replaced the small 12v fixture with a 18w fluorescent fixture. cd is
hooked up as before. however, when the pump is turned on the light
dims and the voltage drops to 10.5 or 11 vdc.

Is this indicative of a short or too many things on that circuit or
does this demand pump really draw that much ? I think, from looking at
the pump switch, that you only turn it off in "emergency" and leave it
on all the time to keep the system pressurized ? Is that right ? When
the switch is off there is no running water. Does that mean it is not
working right , (keeping the system under pressure) or is that really
the way it works ? If you are supposed to only turn it on when you
need water, then it makes sense that it draws so much on that circuit
since it is only used "on demand" and not always on.

What am I missing ?

Thanks for tapping your collective experience.