Having the exhaust moving horizontally over water will not do anything useful for the rocket, the exhaust has pretty much done all the 'work' it is going to do for the rocket by the point it leaves the nozzle. There would probably be some fascinating shock diamond like effects in the air water interface but rocket is gone by that point so the physics details do not matter.
Going vertically the ground effect can have impact for rockets by making them partial ground effect vehicles, but it as per the ground effect wiki page it tapers off quickly based on your pressure generating area, which in a rocket is your engine diameter. So once you are two bell diameters clear of the ground any effect is gone. For serious liquid rockets the back pressure generated from the ground effect causes instability and other problems and is explicitly avoided by having flame trenches/deflectors to produce lots of clear space for the exhaust to move in.
For small (solid) rockets it is possibly to launch from a sealed tube for a slight performance boost, but generally only meaningful when time spent launching is a sizeable amount of the total flight time (aka short range weapons).