Is this a new install? or an old install that just started doing this?
If it's new, could be a few things as others have pointed out, if the battery is most likely good and you're not sure yet if it's in the wiring or it is just the unit causing the issue, then I'd run a temp power connection cable directly to the battery...
Start the boat see what happens, if it stays on, you're batt is good(as we guessed already), the unit is behaving and it points back to the wiring/hookup at the dash.
If it pops off starting up the motor, double extra recheck that the battery is good and properly charging....If double sure the batt is good, the unit is maybe defective(recall, it's directly to a known good battery, has to be the unit)
If it is an old install that just started doing this, then it's probably a corrosion/ wring issues, flaky ground etc, or the battery is tailing off.(as above, run power cable direct to batt and go from there)