My experiance over the last 30+ years---
Do not go smaller--especially if you are hauling dive gear--and a 4 stroke smaller than what you have--good luck getting on plane or much of anywhere.
If you have a smaller engine and you have to run it at full throttle all the time to get somewhere versus a bigger engine that you pop onto plane and pull the throttle back to cruise (which might not even happen with the smaller motor) and burn the same or less fuel.
Also take into account the cost of the new motor--how much fuel will that buy??
I personally would not go smaller than a 150hp on that boat--especially if you are hauling divers and gear. It could just be the old carbed gas hog outboard---in which case--you will have to bite the bullit and buy the fuel--or a newer injected motor(which kills my earlier statement).
You have to do what YOU think is right.
Just remember that our boat motors are not like a wheeled vehicle engine where you go up hill, on the flats and down hill--they are always going"UPHILL".
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 88 V-20
200 G2
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