96 Merc 175 efi constantly beeps when I start motor. Cleaned oil filter, tank on motor and in boat full of oil, replaced cap/float assembly on motor tank. Still beeps. Ideas?

The oil tank must be brim full, or it'll yak at you.
With the motor running on muffs, crack open the oil tank under the cowling until it's ready to leak out. Then tighten it back up.
If that doesn't shut it up, the sensor is probably stuck.

Tank is brim full. If you are talking about the sensor being the float on the tank cap ??, I have replaced that already.

Which doesn't mean it's working. I've seen them not float all the way up out of the package. Make sure it's going all the way up, and the connection is secure.

Pat - thank you. i will check that out

Intermediate warnings on that year EFI are low oil, rotation sensor or water in fuel. If engine oil tank is full I would check fuel filter.Start motor and on front of engine below ECM there is a little black box with a red light if that light is flashing same as warning that tells you there is water in the spin on fuel filter....OPTI

Thanks OPTI