If we let d[t] represent the distance the boat has traveled toward the shore after t seconds the differential equation of motion would be:
d''[t]==-k d'[t], where k is a constant to be determined
The initial conditions are d[0]=0 and d'[0]=30.
Solve the differential equation (I assume you can do that) and you obtain:
d[t] = (30 - 30/E^(k*t))/k, where E is the transcendental number e. Plug in the value of t=10 and solve for k such that
d'[10] = 20 and you find that k = (ln(3/2))/10. So we then have:
d[t] = (-300*(-1 + (2/3)^(t/10)))/ln[3/2]
To find how far the boat goes take the limit of d[t] as t->Infinity and you obtain 300/ln[3/2] = 739.89. So the boat will never reach the shore.