Think I have a couple of solutions to the 'just swim through the water' issue.
1) Build your door frame out of obsidian, and just dump the lava right at the entrance...
<-- placed Lava
2) Remove the water all together, and just carry obsidian blocks on you. place one or two anywhere along the bridge, then undermine a distance away from it, when they break through, just take potshots with your bow.
3) place every other square along your bridge, as part of the floor. Put a pressure plate on each one, and have another entrance that you would actually use. when getting chased, just run along the bridge, and trigger the plates.. Not tested, but you may be able to run over it before it triggers and falls, leaving a broken path for them to follow on, slowing them down. OR.. support the TNT from below, leaving it in place when triggered.. they follow along a mine field.
P.S. First post!