Sounds responsible to me, but two questions, what does RPM stand for (I think I know but want to make sure),
RPM = revolutions per minute... i.e., engine speed in revolutions per minute.
Gasoline Marine generators are governor controlled to operate at between 1500 / 3000 rpm 50 Hz or 1800 / 3600 rpm 60Hz.
(this may vary depending on the manufacture)
This means (unless it has a "warm up" cycle) that when you fire it up, the governor is going take over and increase RPM to their specification.
So don't be surprised if it revs up when you start it.
However, you should be able to reach around and hold the governor linkage and operate it near idle speed..... although these carburetors typically do not have a low speed circuit.
and what happens if one overcomes the RPM regulation,
As said, the seawater pump's volume is RPM controlled. In other words, at the operating engine speed, the pump's volume has been pre-determined.
If you were to force garden hose water into the cooling system, you will risk forcing more water in than what the system is designed for (as per RPM).
does water end up in the oil?
Not necessarily. That would get us into another topic.
I took the exhaust manifold chamber off and it appeared that the water exits the chamber at the same time the exhaust exits
Well, as the seawater cools the engine (via the Heat Exchanger) the remaining and exiting seawater cools the exhaust manifold.
As the seawater leaves the exhaust manifold, it enters what we refer to as a "mixing chamber", where it then enters the exhaust gas flow and out to the muffler.
and I didn't see how the water could overcome the system, except in the exhaust muffler/silence, which I would not use because I have taken the generator out of the boat.
If the generator is removed from the boat, and if the exhaust manifold has also been removed from the generator engine, I would NOT run it.
I would suggest that you wait until the manifold is re-installed, and only after the seawater pump parts arrive and are installed.
Then you will be able to supply Non-Forced garden hose water to the seawater pump's inlet.
Also, since this is a gasoline engine generator, you will want to make sure that the wire harness and controls are able to activate the solenoid shut-down valve.
(this is a needle type valve that closed off the carburetor's main jet upon engine shut-down)
If you are unfamiliar with this and/or are not comfortable doing all of this, I would suggest taking it to an authorized dealer.
The fuel/air mixture must be correct, the governor controlled engine RPM must be correct, and so on.