Re: temp sending unit
Marine/auto are the same. You need to match the sender to the gauge. Senders come in different resistance values. Using the wrong sender with a specific gauge will result in a wrong reading. If you can pull the original sender it MAY have the resistance stamped on it somewhere. I think they are somewhere around 220 ohms.
The other thing to be aware of is the thread size where it screws into the engine. Will be available in 1/8" NPT, 1/4" NPT and 3/8" NPT.