There are two ways to get into the broken bow of Rivet City.
1. There is a door on the bow with a makeshift bridge leading up to it, pick the lock
This is a Very Hard lock so you would need a skill level of 100.
2. There is another door underwater on the side of the bow that is broken
It is close to the starboard side (right side) of the bow if you are looking at the bow from Rivet City. The easiest way to get there is to go to the Marketplace, the exit through a door that leads to a small platform overlooking the broken bow. From here, jump down into the water and swim to the lower right-hand side to a small tunnel to get to a door. I believe this door has a lock level of Easy or Very Easy, so much easier to pick.
Be aware that there are a couple mirelurks on the lower levels of the inside of the bow which makes getting out of the water that much harder. Once you are inside, you will find a switch that will unlock the other door to make coming and going easier in the future.