Water temp vs Exhaust temp
 

Anyone have an idea how much the water temp will effect exhaust temp? I run my boat in Havasu and the water temp will range from 50 to 85 depending on the time of year. Will 20 degrees warmer in water temp equal 200 degrees in exhaust temp?

your exhaust is basically a heat exchanger until the water and exhaust meet. the colder the water, the more efficient the exchange will be due to the temperature differential. the warmer the water your cooling with, the less efficient the heat transfer will be. in a perfect world at 100% heat transfer, the 30* water change would make for a 30* exhaust change but, the with warmer the water, the efficiency drops off proportionally and the real heat transfer rate is no where near 100% to begin with.
assuming your water pressure never changes, i would guess your temp increase is from a leaner fuel to air ratio because the air density change if your gauge is accurate . 200* is a lot though. you may want to check your reading again.

little effect?
 

I would have to "Guess" that a 20 degree swing in sea water would just affect how often your thermostat should open and how much. Seawater will typically range from45- 85 degrees, well below the typical 160 range that your marine engine operates at. You should not see 200 rise in EGT from water temp fluctuation.

If you are running in much colder air than normal perhaps you are jetted lean and this accounts for some of the change in EGT. I know the sleds run much better the more you fall below zero degrees.

The warmer water temperature of 35 degrees should not equate to a large rise in your EGT. When the cooling water rises in temperature to the engine usually means the air temperature rises giving your engine a richer condition not a lean condition which raises the EGT.