Can someone explain any tradeoffs in gear ratio vs prop pitch? Example, adding power to a not ideal ratio (too steep) could be overcome by propping with a longer pitch... But is there some issue that limits this? Cavitation or something? Otherwise they wouldnt sell other gear ratios I guess ?
This can get complicated. To simplify the answer: Pick the propeller design that suits the boat demands, then choose the gear ratio. For example: A Cat with high X-dimension will want more pitch and diameter resulting in lower gear ratio (depending on the horsepower of course). A smaller lightweight single engine v-bottom may like less pitch and a little taller gear (again depending on the HP). A heavier v-bottom may need a little more pitch to help reduce prop slip, and less gear.
These are very basic rules and many factors need to be considered, such as.... The more pitch and diameter, the harder it will be to break the prop loose and get on plane.