Anyone ever wonder why they don just get on an island?
Kinda Long
It sounds like the smart thing to do. There are a bunch of negatives though. The east coast where they are gets hit by hurricanes, snowstorms, and ice. Unless they found an island with some kind of habitat already there, they would have trouble building something to hold everyone and make it through hurricane season and winter.
If there is something already there, then that means there is likely other people to try and share or fight over it. Judging by the series so far, they would have to fight for it. The water can be pretty rough as well and they would have to find a decent boat that could handle the conditions. Depending on how many people and how much supplies are with them, they might have to make a bunch of trips back and forth. So that increases the danger.
What do they do when they run out of supplies? They have to again go back and forth to try and find what they need. All this travel will burn through fuel like crazy and unless they find a stockpile, that is another issue. They better hope to find a boat with a motor and a sail. If everything goes perfect and they grow crops and have medicines and doctors and strong structures to live in, then it could work. Nothing ever goes as planned on this show though.
Since they are close to Washington D.C., they should try and get to the Pentagon. That has to be one of the most secure buildings in the area. There might even be a military presence held up in there still. I think the best overall thing to do is head out to like Nebraska, Iowa, Kansas, etc. and find a farm to live on away from the cities. Unfortunately you need reliable transportation and lots of fuel to move everyone.
They could also go over to the Appalachian Mountains and find a valley to live in. You have the mountains to protect you. I doubt many zombies will be able to get through the mountains. Just roadblock the tunnels going through the mountains. That might be the best bet with their current location. They just have to watch out for the hillbillies living in the area.