1. Return on Investment (ROI)
2. Tourist Videos
3. Further dive training
4. 4k60p standard
...The Rx100iv gives you the opportunity to continue improving your skills and making the connections that may get you that higher-paying work. And you could always invest the money you save into things like an external monitor and video lights, which can have a large impact on the quality of the footage you shoot. If slow motion is what you want, it also gives you great 120fps 1080p and limited 240fps almost 1080p.
Agreed with the points here, filming is a rather closed industry. First one should get a "name" and contacts, maybe some certification from a film school. This is heavy $$, which has to be spent wisely.
First of, as dreifish suggested, I would certainly not start with a state-of-art 4k60p shooting rig at 20k$: learn the technique with a consumer camera, publish great shots on youtube/etc as much as possible. For online, you definitely don't need a 10k$+rig, there are tons of amazing gopro footage out there! Furthermore, your 4k60p camera + housing will worth ~1/5th of its value when you are really going to need it (embarking on an expedition where you are the payed cameraman).
Scuba is a low paying industry with high investment, just take a look, how much dive instructors earn after ~500-1000 dives, probably ~20k$ invested not all for fun. (Maldives is ridiculous, if you have a "life", you definietly need side financing there, which is tedious after 4+dives a day, 7 days a week...) As suggested, I'd also pick up the RX100mkIV in the economical Fantasea housing and put on a WWL-1 and a pair of realistically priced lights from ebay. Important to note here, that handling an increasingly complex system is something one has to learn through many dives in different - sometimes challenging conditions. Obviously you don't want to loose a full 20k rig while filming mantas in the maldives in a 5 knot current.
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