I have a 96 502 with the stupid sea water/fuel pump all in one..and I noticed today that my fuel pressure is only at like 4 psi..I believe that the pump is starting to take a dump. So I want to just remove it and put in a electric fuel pump. Question is..what can I do with the sea water pump now cause it has the pump attached? Other question is, how can I make it so the pump kicks on without having a switch on the dash. I know I had to have a fuel pump switch on my outboard..but I don't want to do that on my boat now.

Get a low pressure oil switch. Then wire it into trigger the fuel pump relay. Best way to do it cause if you use a toggle switch on the relay there is potential to forget to turn it off. In this case it is possible for the fuel to leak past needle and seat and hydraulic the motor...

I did the same thing on my 525SC...you can get a block off plate at about any auto parts store. As far as the wiring, I ran it to the fuse block that had switched power and put in a 15a fuse. Make sure that you put a regulator and pressure gauge on. Also...mount the new pump as close to the tank as possible and before the filter.

You have to run a relay off of you purple ignition wire. When you turn on the ignition it then powers up the relay. You need to have the electric fuel pump wired to the battery or a positive terminal.
Terminal 86 will be your purple ignition wire. They have it labeled "Red fuel pump power wire"
http://www.auto-nomics.com/faq-fprelay.html

Also mount the fuel pump below the tank if possible.

I just went through this exercise and ultimately decided against a new electric fuel pump. Each of the two fuel pumps on that engines are available on the market for much cheaper than merc wants.

Here's the link to the mechanical pump you need, even has the ports in the right spot to reuse the tubing- http://www.amazon.com/Airtex-60932-M.../dp/B002HTYRD8

If your going to the effort to take the old pump off and put a block off plate on, why not just replace the mech fuel pump and be done with it?

Cause this style pump is crap and known for failing. Its known for leaking fuel into the oil in the pump causing the pump to fail. Its not a mechanical pump that runs off the fuel boss on the cam its turned buy a belt from the crank pulley.