I can't say for sure, but I suspect your boat (1998, 4.3L) has the same set-up as mine (2001, 4.3L); at least the wiring diagram you refer to suggests it.
There are 2 oil pressure senders on this motor. One is located back by the distributor and is a barrel-type; there's some brass plumbing connecting it to an oil passage in the block, and it has a single blue wire with a washer-type connector sandwiched between two brass nuts. This is the sender that is connected to your dash/gauge; variable resistance gives a reading on the gauge. If you want to confirm oil pressure, this is the easiest place to add a mechanical gauge, as all you'll need is a "T" brass threaded connector and a $10 gauge from any auto parts store.
There's a second oil pressure sender, often called an oil pressure switch, which is screwed directly into the block. I have seen some that are down by the oil filter, but mine is the first thing I see when I open the engine compartment (sorry, the boat is stored off-site, and I can't go and look at it right now. As I recall it is forward of the air cleaner/spark arrestor, and roughly the middle, side-to-side.) This one has a Tan wire with a spade-type connector, and looks like ones on my GM cars of the same vintage
http://www.rockauto.com/getimage/getimage.php?imagekey=3272901&imageurl=http%3A//www.rockauto.com/info/KEM/PS139.jpg. This switch sounds an alarm in the engine compartment if the oil pressure is below 6psi. When you turn the key to "on", you should hear an alarm on the dash, and one from the engine compartment. When you turn it to "start", once the oil pressure gets above 6psi, the alarm in the engine compartment will stop.
I'm not aware of a safety switch that will disable the ignition if either of these senders reads 0, as they will both read 0 before the engine starts and the oil pressure comes up. Such a switch would make it impossible to start at any time.
So, finding this oil pressure switch is unlikely to help your no-start problem. The oil pressure flashing on the dash might simply be because the engine isn't firing, and the starter isn't able to turn the oil pump sufficiently to build significant pressure, so it's correctly registering a fault condition of low oil pressure.
I'd chase down your original thought of fuel. This should be a carburated engine, correct? If so, remove the air filter/spark arrestor and have a helper pump the accelerator a few times. Do you see fuel being sent into the carb? Can you confirm fuel pressure? Maybe a blocked fuel filter? I'd rule out the easy stuff first.
I feel for you. I'm responsible for maintaining our fleet of vehicles at home (7 cars including two from 1955, the boat and 2 dirt bikes); the boat is by far the most stressful because there's only 1, and if you're out on a lake or river and something happens, help is a lot farther away. Good luck.