Maybe I should clarify the situation on my boat. The bilge pump wiring leaves the DC panel, the breaker is powered independent of the main switch, down to the current bilge pump/float switch combo. When the diaphragm bilge pump replaces the current centrifugal it will be mounted in the engine space. There are really no other alternatives for mounting. This means the wiring would now run from the DC panel to the float switch in the bilge and then back through the same spaces, past the DC panel, through the battery box area, and into the engine space to the diaphragm pump. The wiring run will be quite long and since it essentially doubles back to its source the situation is ideal for the possible use of a relay. Given that it takes @100ma to close a 30amp relay the bilge wiring could be quite compromised and still drive the relay without voltage loss, etc. to the pump. The pump wiring remains high and dry out of the bilge. Wiring and installing a relay in this application is very simple as it would be right behind the DC panel.
Bilge pumps produce some interesting ideas/opinions. I feel that trusting a bilge pump to keep your boat afloat when you are away is useless. If there was a catastrophic failure while the boat was on the mooring at 2am, unless the alarm was extremely loud, the batteries would be depleted and the boat would sink long before anyone noticed. I just don't believe in any battery based system lasting long enough. For routine rain water/packing drip the pump/switch combo is great. The situation is entirely different if you are on board. I'm okay with not even bothering with a float switch on a large backup "disaster" pump. If you are on board then flipping the switch is simple enough and leaves the notoriously unreliable flaot switch out of the system.
So am I sure I want to use a relay in my system? Not really. Its an idea I wanted to explore. I understand the pros and cons of complexity having fought through it with my charging system. So we will see..... Depends on how many amps the final pump choice requires and how much I want to spend on a bilge pump switch. Since I have changed Rule switches as frequently as socks the expensive switch is tempting. We'll see.
RT