Lynn Coffelt wrote:

Quickly, this isn't going to be any help, but once an SCR fires, (in a
DC circuit), it isn't easy to halt it's conduction without interrupting the
current flow by introducing another switching device....

Well, yes, that can be a problem, but I think SCRs can be shut off by
changing the bias of the gate circuit. They have to have a minimum
current flow to work. Trouble is, all my semiconductor experience was
with transistors, diodes, and ICs, and I cannot find much about their DC
application on the net. (And whatever twit named a series of boats "SCR"
is begging for a kick in the shin.)

The other (non-helpful, I know) question is that if there is backflow
from the discharge hoses, shouldn't there be checkvalves to prevent seawater
from entering (especially during a moderate heeling turn?)

The check valve would have to be right at the pump, or close to it, to
keep the water flowing back out of the hose from floating the switch.

I'm wondering about the same pump control techniques, but this is at my
house when we have more than a little rain. Like this past week!

Should work fine in a sump, where the hose emptying back into the sump
would be even more a problem.

You are talking about a house, so you have a bit more room for spare
parts. Here's what I figure for a latching relay circuit (if you don't
need it, sure as anything someone else is going to ask):

Need:
Pump, relay with at least two normally open switches, two float
switches, bunch of wire.

Radio Shack has at least one 12VDC relay, and at least one 120V relay
that will work. Watch out, they look exactly the same and you have to
check the label on the package. (They are DPDT.)

Place the upper float switch in parallel with one of the normally open
switches. One side of them goes to one side of the power, the other goes
to one side of the relay coil. The other side of the relay coil goes to
the lower switch, and the other side of the lower switch goes to the
other side of the power supply from the first set of switches.

The remaining normally open switch controls the pump.

I think the Radio Shack relay is a generic automotive module, and I know
I've seen bases for it around somewhere, so you could solder the float
switches, power supply, etc to the base and just plug in a replacement
relay when the old one goes bad.

Of course, it would be handy to add a third float switch even higher
than either of the first two, and wire it to an "OH,****" alarm G.