The speed boat captain is likely guilty, for different reasons:
1. Must adapt the speed to the visibility and be able to see any navigational hazards such as shallow bottom, rocks/reef, moving boats or floating debris.
2. Must overtake all boats to starboard of the slower boats.
3. Must have at least 50 metres between two intersecting boats or for overtaking the slower boats.
4. All slow or less maneuverable boats have priority over a speedboat with two or more engines. So in this case the speedboat should have driven away from the tanker vessel.
This is what we learn and use for sailing boats around the world, including Thailand, but with more caution in Asian countries.
Seafarer Divers Charlie Phuket