Gages and sending units generally can be bench tested. The big question is what meter readings are within manufacturer's specifications. A word of caution: some delicate electronics may be harmed by just the current from an ohm meter. Having said that, you may be able to trace down manufacturer's specs for some of the components. The internet has some amazing info, and a lot of worthless info too. With others you may be able to determine if the gage or sensor is functioning without having specs. Your testing may not be completely accurate, but you may be able to eliminate some components during your troubleshooting. You didn't mention the type of boat, but keep in mind that fiberglass boats require a grounding system. It sometimes helps to run an additional ground wire from the negative terminal of the battery to the bus bar, and make sure that all hot and ground connections are good.
Keep in mind that if you can not fix it with a hammer, it must be an electrical problem.
But then again, Some electrical problems can be corrected with the use of a hammer.........and a trip to the parts store.