How much larger do you need the speaker hole? Is it in a location that you can attached a template? We need a little more information and a picture or two would be great.

 

Welcome to the forum.

 

If you cannot use a template you might want to use a flush trim or template bit (depending of which direction you have access) with am undersize bearing.

You can build one in your own by just getting a new bearing for an existing one....or
12v Tools | Router Bits | Tape | Templates | Sets | Bearings | Adhesives these guys have sets specific for this purpose.

M

 

If the hole needs to be perfectly round then you will need a template. Depending on where the cut will be made it may not be possible to do it with a router. It may be easier to attach a circle of wood to cover the existing hole with hot glue and use a jigsaw to cut the hole bigger.

 

Tony - welcome to the forum.
photos of your project may help with more accurate replies.
on a boat, nothing is as simple as we would like them to be.
for a speaker hole, in my opinion, it does not have to be perfectly round if you are going to insert a speaker then a cover on top of it - the hole will never be seen.
so some photos would help us help you.

 

Carbide tip bit will work fine. If you just slightly enlarging freehand will be fine, if it's a lot cut it undersize about an 1/8" with a jig saw then use the router bit. If you can't use a jig saw cut it undersize with the router then come back and clean cut it to the line.
If you want it perfect you could make a template, as mentioned above, and tape it in place if there is room.