How much battery power do i need for my 16 foot boat?

I want to get a new battery I found a 31 group I want to put in my center console will that be enough power for start my Honda 50 and run small gps and fish finder and run electric down riggers and I have a horn, bilge pump and led nav and led interial lights now I have 2- 27 group batterys one for the Honda after one day its used almost no power and the other will be low for the rest my batterys are from 08 so its time to get new ones it would be nice to have the deck space and only one battery. What are others doing with their set up?

Battery "groups" are only indicative of the physical size, not the actual capacity of the battery. In most cases, bigger batteries have a larger capacity than smaller ones, but this is not always the case. You need to look at 2 ratings of batteries- MCA (marine cranking amps) and AH (amp-hours capacity).

The problem with smaller outboards such as yours is that they have a very low charge rate, for example your outboards' charge coil only produces 10 amp at 12 VDC maximum, and much lower rate of charge at idle speeds (about 3-4 amp at 1000 rpm). I would first test your outboards charge rate as follows: start with a fully charged battery- use a voltmeter on the battery posts. At idle, you should have a reading of 12.5 VDC or higher at idle. As you increase rpms, the voltage reading should also increase, up to a maximum of 14.9 VDC. Readings outside of these limits indicate a charging problem with the outboard.

If your outboard is charging OK, I would suggest a 2 battery system with a selector switch. The selector switch will allow you to use and charge 1 battery at a time (charging 2 depleted batteries at the same time can cause problems with the outboards charging system- failed rectifier or charge coil). For your application, I would recommend using AGM marine hybrid batteries rated at 600-700 MCA. AGM hybrids have the power to crank your outboard to start it, and have the capacity to run run other devices. AGM batteries have a much longer life than a standard VRLA battery, and can be deeply discharged without any damage to the battery. I would also recommend that you install a "battery tender" charger.