Mercury 90 hp Optimax problem

many mercury motors use a rubber mounted prop on the shaft which is a sacrificial idea to prevent gear box damage and prop damage in with under water objects
what you suggests indicates that at certain load situations that rubber has failed and allowing the shaft to turn but is not driving the prop as required
It is driving just enough to give you steerage to get home

have you replaced your fuel filters recently

A few things will cause this, oil injection is too lean, fuel mixture is lean, water intake restriction. make sure the oil tank is pumped up, many overlook this simple step. Make sure water is coming out the water check hole. (the small stream of water coming out to show that water pump is working.

here you go

Mercury Optimax 115 Service Manual

it appears to be a fuel starvationproblem
it can be from the tank vent still locked close
fule lines collapsing internally
fuel pump diaphragm fai;lure
blockage in the main jets
even tye hand pump bulge in the line can be failing and blocking the fuel lines

start by going over ignition system and fuel

your carburetors might be dirty blocking flow to the high speed jets causing it to bog down

Sounds like you need to service your carb as the mainjet is probably partially blocked or you have a choke thats not opening properly or a blocked filter.

i have one doing the same thing the choke is all the way on and it is warmed up but when you give it gas it goes for a while then boggs out