which will the best motor for boat " http://www.hobbyking.com/hobbyking/store/__8595__2040SL_4800kv_Brushless_Inrunner_WaterCooled_.html" or

"http://2.imimg.com/data2/QD/EP/MY-3427831/40000-rpm-servo-motor-250x250.jpg"(40000 rpm)

please help me 

That will all depend on what boat (dimensions / design) you are trying to build, intended performance and run times. I have used similar brushed motors on several boats without any problems.

The brushless motor is ideal for higher performance crafts like racing boats.

BL motor is for high performance, but you can also use it for slow boats, doesnt matter