Placement of limpet mines depends on the capability of those placing them. Ideally they are placed as near to the keel as possible for maximum effect. If all you have is a speed boat with no dive capability on board you place them where the water line is at or below as conditions permit. If the tanker is empty the water line is much higher than normally loaded.

A 'good' rifleman with a scoped rifle would solve a bunch of these kinds of problems. Oil companies are too cheap to hire them though. Would rather depend on the US to handle it one way or another. Also the UN frowns of folks defending themselves. Some countries are stupid enough to agree with them.

An 'accurate' 50 caliber scoped rifle could hold off speed boats at distance. Could take out both men and motors. The Barrett is NOT accurate enough. I would use a bolt action McMillan or a SERBU semi auto. If you want to go cheap 50 cal, then I would use a magazine fed Safety Harbor top end unit on an AR 15 lower. It IS accurate enough out to 2,000 yards. Each ship would need at least 2 qualified riflemen to handle 12 hour watches while in the danger zone. Other times just be on call.

This would also require a world wide ban on speed boats being allowed within 2,500 yards of either tankers or freighters unless for legitimate reasons like the putting on or off of Pilots or the coming and going of crew in port. Anything inside that would get ONE warning shot, then you start killing with no crap or penalty from anyone. Would be able to enforce it by declaring ports or even whole countries CLOSED to all commercial traffic until the country does agree. Thusly nothing IN or OUT trade wise by sea. Sort of what is in effect with airports today world wide.