Beej67 is almost right
Always remember to start at the source and work foward.
No water comming in -- no water going out
No fuel in the tank -- no fuel at the injectors
Low voltage in the battery -- low voltage everywhere
Don't get caught up in the, "it happened to me once and it was the---".
Trouble shooting is a step by step process and if you skip a step and you can't fix it you have to call in a pro.
He is the one that plugs in your dock line and fixes your electrical problem and ONLY charges you $50 for the call
Rule one still applies.
The last thing you mucked with is the first thing to mess up.
Before you do anything, check the 'dumbass' factor and make sure your thru hull valve is open. I've been guilty of that before. Hehe. Now, on to narrowing the problem down:
Detach the hose that goes from the heat exchanger to the mixing elbow, then start the engine for 20 seconds, maybe less. Does water come out of that hose?
If so, then your problem is in the mixing elbow.
If not, then reconnect it, and detach the hose from the raw water pump to the heat exchanger, and start the engine again. Does water come out of that hose?
If so, then your problem is in your heat exchanger.
If not, then leave it detached, close your thru hull valve, detach the hose between the raw water pump and the strainer, and stick that hose in a bucket of water. Start the engine. Does water come out the back of the pump?
If not, then the problem is in the pump. If so, then the problem is in the strainer, or between the strainer and the thru hull, or there's some gunk in the thru hull itself that's clogging it.
NOTE: do this in short bursts, while the engine's cold. Don't go and overheat your engine and then say "beej67 told me to do it!" Hehe.