@mamamouchi
Very preoccupied with 'cathedral' naming in Philippines it seems. Just to complicate life, there's also a tiny island 'Blue Cathedral' off Siargao at approx 9.844899 126.157463 about 1.5km NW of the dive site Blue Cathedral at 9.834645 126.167505:

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I'm so sorry, like tli i have no idea how to help you further. Caty's advice is good i think and you should contact other local dive shops/operators to check coords of the questionable sites. I recall dimly there's a problem using Standard UTM conversion outside continental NA as i said at first. This is something to do with a particular and area-specific equation used in converting there which is not usable for the rest of the world, however i thought the likely error was more that of a few [or at worse several] hundred metres, not many kilometres as you're finding.

Here's another thing: When i convert the UTM coords [52174139E;1077749N] as you're supplied in P#9 to lat/long it places Blue Cathedral dive site at 9.736867 126.030315 and surprise, surprise! You'll never guess where that is:

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Yep, pretty-much right back where we started from at what you thought was a dive site ~200m directly to the east of the point of East Bacus Grande. Maybe you've accidentally mixed things up somewhere.