88 v-150 wont idle in gear in water

have a 1988 v-150 merc that will not idle in the water. with muffs on idles fine. in water hard to start hot unless fast idle arm is raised. out of water idles fine as soon as i touch the key. my mechanic is telling me that the problem is todays fuel. i need to rejet my carb. anyone know what increment to use. had .048 in it. went to .050. no change. he says its back pressure from the water causing the difference. when moving at a slow speed and bumping up the throttle it will smooth out. when i hit it, runs fine at all other speeds. i've seen alot of other motors with this problem online. no one has suggested this fix. all the common things have been repeatedly checked. compression plugs wires coils switchboxes etc. gets time consuming and expensive to buy a bunch of jets. hope someone has an idea on this. thanks ahead of time

My Merc 115 HP starts and runs better on Shell or Sunoco 93 octane.