My boat has a temperature warning light. Can I fit a gauge to replace it?

The short answer is yes - but there is a long answer.
Below you will see three pictures:
Top - a sensor that runs a warning light and a gauge.
Middle - a sensor that runs a warning light only.
Bottom - a sensor supplied with a Gauge that is matched to it.
The sender fitted intended to drive a light will not drive a gauge. so this gives you two options.
1 Purchase a gauge AND a matched sensor that will drive both your existing lamp and your new gauge and replace the original engine sensor with the new one.
2 Purchase a gauge AND a matched sensor and replace the original engine sensor with the new one.

Temp & Warning light sender

Warning only sender

Gauge supplied with its own sender