From Disney's website:
<b>The Magic Kingdom® Park is accessible by water launch from Disney's Contemporary Resort, Disney's Grand Floridian Resort & Spa, Disney's Polynesian Resort, Disney's Fort Wilderness Resort & Campground and Disney's Wilderness Lodge. The Disney ferryboats connect the Magic Kingdom Park to the Transportation and Ticket Center (adjacent to Magic Kingdom parking).
Epcot® and Disney-MGM Studios are accessible by boat from Disney's BoardWalk Inn and Villas Resort, Disney's Yacht Club and Disney's Beach Club Resorts and the Walt Disney World Swan and Walt Disney World Dolphin Hotels.
Downtown Disney® is accessible by ferryboat from Disney's Port Orleans Resort and Disney's Old Key West Resort.</b>
It's possible to go all the way from MGM to Epcot by water taxi, but it takes a fair old time (MGM - Swan & Dolphin - BoardWalk - Y&B - Epcot International Gateway).
The marina at DTD is at Marketplace near Cap'n Jack's. You used to be able to travel by boat from Westside to Marketplace (and vice versa), but I haven't seen this in operation for a while.