DO NOT use "teflon tape" DO NOT use any type of RTV, or "gasket maker" or "pipe sealant." This part of your exhaust system is way too hot for these kind of products. This is all very bad advice
. Many of these products give off VERY toxic fumes when overheated.
sealant is one answer, and it will stop the leak, for sure, but you will have a nightmare getting those parts
separated in the future.
Use a copper (or copper/nickel) based high temperature anti-sieze. (lots of brands!) It will seal the leak (Assuming you actually have properly mating threads), and give you a fighting chance of getting the threads apart in the future.
You should have some of this on the boat
anyway, it is the right thing to use on engine
nuts and bolts to be sure you can take them apart next time.