The point of the sea, is that it needs to be somewhat close by for the boats. Closer towards the end of the RP the goal will shift to guarding the boats under construction outside the city walls so that when they are complete as many people as possible can be evacuated into them and escape in search of a land without titans. Sort of like
Part 1 - Defend the outer wall
Outer wall falls
Part 2 - Defend the central wall
Central wall falls
Part three - Defend the inner wall
Part four - boats are beginning to be constructed not far outside the city walls. Protect them from titans.
Part five - Escort citizens through the remains of the city to the boats.