Charging Handle, having served in the same AO maybe I can help a little. There were 3 Navy task forces serving on the Mekong Delta river system.
TF-115 was the PCF's, known as Swift Boats, which served primarily along the coast to interdict VC and NVA resupply coming down via the waters surrounding Vietnam. They also entered the rivers from time to time. They were called the Coastal Surveillance Force, and "Market Time".
TF-116, the River Patrol Force, was the PBR's you mentioned and their primary job was to deny the enemy the use of the rivers for troop movement and resupply. That's why they were called the "Gamewardens" as they spent many hours stopping sampans to check for contraband.
TF-117 was the Mobile Riverine Force, which was a unique combination of Navy assault boats and support ships and elements of the Army's 9th Infantry Division. Their primary job was to use the rivers as a water highway to seek out and destroy Charlie throughout the Mekong Delta region.
All three task forces were involved with SEAL operations, however I believe the SEAL's were primarily attached to TF-116. The various boats and ships of all the task forces were used for SEAL insertion and pickup, along with fire support.
Boats from all three forces used twin .50's but you offer a very good clue as to which group your uncle may have served with. Dong Tam was the main base used by Task Force 117, the Mobile Riverine Force. I doubt if there were ever any PCF's there, as the base was a hundred or so miles inland. There were definitely PBR's there from time to time but I don't recall any permanent detachment stationed there. There was a detachment of SEAL's on the base.
If your uncle served with TF-117, there are several types of boats he could have served on. I don't recall the troop carriers (ATC's, or "Tango" boats) having twin .50's, but rather they used 20mm and 40mm gun turrets. I believe the Monitors, CCB's, Zippo's, and ASPB's may have all used twin .50's, along with 20mm, 40mm, and even 105 howitzer's, but not all boats of each type were armed the same.
The web site mentioned by Kaiser Soze is a good one. I had the pleasure of meeting Kent Hawley at the Mobile Riverine reunion this past June. However, there is a Mobile Riverine Force Association web site located at [url]http://www.mrfa.org[/url] with much more info.
If you think your uncle was part of TF-117 I can recommend a good video which will give you a very good history of the unit. "Force Afloat" is available from the MRFA or can be ordered through Kent Hawley's web site (look towards the bottom of his site). While you're at it, email Albert Moore (on the MRFA site) and find out if your uncle is a member. If not, he should be. They accept members from TF-115 and 116 too! With all this information I'll just bet your uncle will be surprised at how much you know and will willingly open up about his experiences.