Here's a picture of my sending unit for my temperature gauge installed where the brass or sometimes nylon plug mentioned by SkiDad is usually located on the 4 cylinder Force motors. The red wire goes to the sending unit.
When selecting a temp. gauge, be aware these motors run much cooler than an automotive engine. Around 130? - 145? F. Most automotive gauges start at around 120? F. If you use a gauge that starts at 120?F, and your Force outboard is at normal running temp., the needle on the gauge will barely move. I found a gauge that starts at 65? F and goes to 200? F, so at normal running temp., the needle is pretty much in the center of the range.
You can run the motor without a thermostat, however, you won't get maximum power or efficiency as the motor needs to be hotter in order to do so.
