A couple of thoughts:
1) There is ONLY two places you should be drawing power for your 997 unit - direct from the battery or direct from the circuit breaker box if your boat has one. Anywhere else and you'll probably have an issue.
2) If the battery you are running your unit is low, your unit may cut out. I know that happens if I run my 997 off the motor (starter) battery and go to start the engine while the unit is running, it will cut out; but only if the start battery is too low. I believe the Humminbird has built in shutoff protection for low voltage...otherwise, consider #1 above.
3) You can connect your power cable from the 997 to your circuit breaker, if your boat has one, just like a house breaker. Or you can connect directly to your battery. In either case you need a 3A fuse in the line.
a) If going to the circuit breaker, you will have slots filled with or open for push/pull fuses. Your red wire from your unit should be connected to a post next to that 3A fuse with a female slide-on connector. Your black (ground) wire should be connected at the base of your circuit breaker box where there's a whole bar of ground posts that you can choose from to push on a female connector for your black wire.
b) If going to the battery, you should have an inline 3A fuse. You can buy these at NAPA.