Rate of work done against water resistant? can someone explain why its 3.0w? .

You understand the concepts all perfectly fine, just have applied them slightly incorrectly.
Firstly, the speed time graph is actually a triangle since the question says a steady acceleration from rest - i.e. it started at 0 and worked up to 2ms-1 in the space of that 10 seconds. The area under the graph is the distance, and a triangle area is found with bh/2, and in this case that is 10*2/2 = 10m - so the boat travelled 10 metres.

Work done as you correctly stated is fd - giving the part against water resistance as 3*10 = 30J
However, you forgot to do the last part - the key words are rate of work - this means Power. Power is Energy/Time, and this whole process occurred over 10 seconds. So whilst 30J of energy were expended due to the water resistance, this didn't all occur in no time. So we need to divide that by 10 to get the power. And 30/10 is 3J/s - and J/s is the same as W.