As an update re Turkish Airlines allowance - make sure to get something in writing (like an email) when you call them to arrange that extra luggage and confirm it has been added both ways! Also do keep tickets and baggage tags from your first leg.
We have originally called, and were assured all is well, 2x 30kg (in addition to the standard check in allowance) was added , but we will not receive any confirmation and it wont show on the ticket. On the way out, from Germany, it was all fine - just mentioned diving luggage and was allowed to check in 4 bags, almost 110 kg in total for 2 people.
However on the way back, in Manila, the fun started - first they refused any diving luggage, then upon literally opening Turkish Airlines website on a laptop and shoving it in their faces, they were adamant it is part of (and not in addition to) checked luggage and were very keen to levy excess baggage charges (not even sports baggage charges, 90 Euro for the whole bag up to 32kg, but instead excess counted at about 70 Euro per 3kg) claiming also diving thing does not apply in Manila because Manila is so special. Took 50+ minutes arguing first with check in person, then supervisor, then manager (got to be a Karen alright xD) , and then she finally called somebody up who seems to have resolved that as she did check in the luggage straight away and walked away with a sour face without uttering a single word. What I think helped to convince them was also the fact that we had our first leg tickets with all the baggage tags, hence were able to prove that the bags were accepted back in Germany without any issues.