Common for the fuel gauge to bounce with half a tank. Rougher seas and it will bounce more. You did not say what engine you have but yes carburetors were used well into the 2000ís. Although fuel injection was available as an option. Do you have a regular carb with a choke or a TKS that uses an enrichment circuit?
Remove flame arrestor and if choke style there will be a butterfly plate on top.
1997 2400 SCR
5.7 Vortec / Bravo 2