Check the voltage levels at the devices, both when idle and when operating. If there's any dip then you have something to check. Start with removing and cleaning the connections to them. A bad ground or corrosion will cause them to consume more power than usual. A device suddenly draining a lot more power can cause a dip that affects other devices. Pumps that are failing will likewise consume more power than usual.
If you've got a portable clamp-style amp meter you should use that to check how many amps the pump is pulling. Compare that against the vendor's specs for it. If it's more than spec then the pump could be suspect.
That and if the wire gauge isn't large enough it'll contribute to voltage drop. Boat wiring is notorious for under-sized wiring. Both on the positive and negative sides.
Assuming the you've checked the pump amp load, check the voltage level at the battery. Then check it at the pump (while it's not running). If there's any more than a ~.3 difference then the wire gauge might be a problem (assuming you've already check/cleaned the connections). Then check it while the pump is running. If it dips then it might be worth considering replacing/upsizing the wire to/from the pump.