Increasing the capacity/size of the battery pack, does not nessessarily mean you need a bigger/heavier cable harness.
Upgrading you battery pack to 9.5 Ah means it will run twice a long on the same current draw or twice (approx) the current for the same time. Are you changing your lamp (bulb wattage) also? That may require a heavier cable connector rating.
If you are planning on runing a higher wattage bulb (lamp)
Simply divide the wattage of bulb by the voltage, and that will give you the current (amp) rating you need.
A trailer plug rated at 8 amps assumes a 12volt supply (automotive). 8 x 12 =96 watts. So unless you planning on using a lamp over 96 watts rating at 12 volts, the 8 amp rating is just fine. Incandescent lamps (halogen) do have a high start current, but this is very short duration (seconds) and will not be a problem underwater. (cooling).
Mike D