I'm pretty new to boats even though I've been flying nitro RC for 15+ years, but electric boats are different. I got a good motor, ESC, and hookup for twin batteries in a Lobster Boat I'm putting together, but my question is do I need to insert a fuse in the circuit? I used to race rc cars with my sons many years ago and I remember a fuse being inserted and every once in awhile it need to be replaced. Do I need a fuse for boat use, and if so, where does it go in circuit? Thanks....

if its a newer esc then it won't need one.
Your lobster boat ?
who makes it ?

My kit is the Midwest Lobster Boat and the motor etc was from MACK systems. I figured this would be cheaper than a long distance call as they don't have email. Its a newish flat blue ESC, can't recall the brand. I just wanted to make sure about a fuse, so your saying the new ESC can handle voltage spikes etc. Thanks...

Newer ESC's are pretty good, but me, I would still put a fuse on the battery lead. Can never be too careful. All it takes is one short to make a bad day.

Does it make any difference which side of the battery, black or red? What size fuse is safe?

Go for the 'red' side of the battery (+). If the whole system is done correctly the fuse will never be used. Which is good! I'd rather buy several fuses rather than an ESC, etc.
- 'Doc

Depending on the ESC, most can take 30 amp surges. That should be enough as the Midwest Lobster boat isn't made to be a speed demon.
If it was many years ago since your kids raced R/C cars, they might have had the mechanical rheostat speed controls. Some of them used fuses. Futaba ESC's had them built-in and possibly still do.