Operator and his son launched V1 a 20 foot Tracker Marine L.P. pontoon boat from 9257 July LN St Augustine FL to test the engine in preparation to sell V1. 

V1 rolled onto its port side and was kept half afloat by the starboard pontoon and began to float south on the ICW.

The port side pontoon came to rest on the west bank of the ICW at channel marker 90.

witness witnessed the pontoon separate from V1 and told his boss what happened.  

Boss entered his vessel and responded to the scene to assisted V1's occupants from the water. Boss attempted to tow V1 to shore to get it out of the channel. V1 began to drag the bottom just south of channel marker 90 and Boss could not get V1 out of the channel to shore. Boss transported Operator and his son to Marinland Marina.

 Operator contacted Sea Tow to remove V1 from the water. 

Conclusion 

Based on evidence, statements and information gathered to date, I have concluded the following:  
There were no violations of Federal Navigational Rules or Florida State Statutes.

A possible contributing factor that caused the accident is deterioration of the bolt holes on the aluminum cross members where the pontoon was connected to the deck.
MARINELAND