I have a stereo I want to install, but I'm not sure how to wire it so its off when the ign is off.  Any help/tips would be great.

On the back of your ign. switch, there should be 3 poles.  One says Bat, one says St and the other says Ign/Acc  You connect to the Ign/Acc so accessories are one only when the switch is on.  Bat is always hot, and the ST is for starting only.  Hope that helps.

Right on, good info in here.    You guys are hired!

As long as the stereo is aftermarket (Sony, Kenwood, Alpine, etc...) and not a factory stereo (Ford, Chevy, etc...) then the red wire goes to Ig or Acc on the back of your ignition switch. Some ignition switches have a ACC pole and some don't. If your ignition switch doesn't have a ACC pole the red wire goes to the IG pole on the back of the ignition switch. Yep, yellow gets power all the time.

BTW, most stereo yellow/memory leads consume very little power and your battery should be good for several months.

If your stereo is a factory stereo repost with a pic of the stereo and I'll tell you how to wire it correctly.

Where are you guys hooking up grounds for amp(s)?

Sorry for the Thread Jack

Directly at the battery.

I figured it would be there or to a motor mount.

Anyway to ground it up front? ...

My 20 years of experience have taught me to ground a stereo amp at the battery to avoid future problems. The ground wire must be the same size as the power wire. Bigger wire is better wire. If I install multiple amps I run a huge power and a huge ground wire to distribution blocks that are mounted close to the amps. The size of the amp(s) will determine the size of the power and ground wires. IMO 

Ive run them in cars and I thougth the standard rule was the shorter the ground wire the better.  Course in a car there are 54 thousand places to ground, the boat, not so much.....

That is the impression that I was under, but in a boat we are SOL.  So, like Rod said, make sure that the ground wire is the same size as the power wire.