Would a boat move faster in warm water or cold water - say 85 F vs 35 F? Also; faster in salt or fresh water?

Well we know that in the two examples U give, the DENSITY of water is different.

Hotter water is LESS dense than colder water, as given by the fact that "hot water rises".

&

As substances are dissolved in water (like salt) its density INCREASES.

If we take for example, a boat that expells water in order to get motion,

the change in momentum of the water being expelled {to the rear} gives the boat a forward impulse, acceleration, and speed.

This momentum change would increase by the water becoming MORE dense - IF the power of the engine was increased with the water density so that the velocity of the water being expelled remained constant. Then momentum change and impulse would increase along with boat speed.

So IF boat power changed with water density accordingly => the boat would move faster in colder water (more dense water) and in salt water for the SAME reason - GREATER density in both cases.

{Note: IF the boat's engine throttle (output power) was held constant, rather than increased as per above, then the boat's speed would decrease in DENSER water, since it takes MORE power from the engine to keep the same forward speed (ie same impulse) as in less dense water.

Well, I'll give a complete guess at this with no scientific research that I know of to help me out....I'd assume that there would be a difference but, completely unnoticeable, if the water is more salt than fresh we then know it'd naturally be more buoyant therefore the boat could generally get as little contact with the water as possible creating less drag than fresh water....In terms of temperature, firstly, if you really need a boat I guess it's safe to assume it's a large area therefore the temperature would be different every 5 to 10 foot per square area....If heat naturally rises I guess this could create the most mimimal of "lift" again creating less drag although again this would be mimimal....If naturally heat rises then perhaps debris under water could be closer to the surface then normal and perhaps clog the propellers. although this is unlikely since they'd be designed to deal with this....So, my view....Yes, this would make a difference but, would be completely unnoticed.