[Daniel101]

Pay can be pretty shoddy for diving instructors. In the UK, you can make a living/pay the rent. Abroad, the costs and/or package (accommodation/food etc) will allow you to live at that location - but you wouldn't be able to save very much and are unlikely to be able to pay any mortgages etc back at home.
Many resorts will pay a low basic, followed by commission on the courses you teach and any equipment sales that you make. The commission structure means that you have a vested in interest in getting your students to progress in their continuing education (con-ed)...so there is a strong element of selling involved.

The more qualifications that you hold, the more you can offer - and the more you will earn. Being OWSI is the bare minimum. Learning to teach specialty courses and getting your MSDT will open up many more options for you...and add some variety so that you are less likely to get bored running an endless series of open water courses.

Some locations have other advantages. For instance, in the Caribbean you might earn big tips from US tourists. That is less likely in South East Asia.

It is also very valuable to pick up some extra dive-related qualifications....

Firstly, get some experience running and maintaining compressors. If possible, get a gas blending qualification so that you can pump nitrox (and trimix).

Secondly, get some experience maintaining dive equipment - if possible, get qualified as a service technician for one or more major brands (Scubapro, Cressi, Mares).

Thirdly, learn to speak some other 'core' European languages; i.e. German, Dutch, Swedish etc

DO NOT underestimate how much it will cost to gain all of these qualifications. It costs A LOT. If you are going to do an instructor course - do it with PADI. They tend to offer the most jobs and it is the only agency that won't accept cross-overs. So, once you have trained PADI you can simply do a cheap(ish), short cross-over to NAUI, SSI etc. You can't do it the other way around.

Learning and gaining experience in England is a good way to do it. You can hold down a job and complete the DM & IDC in your spare time. After that, move up to MSDT by working in your free-time. Many people work freelance, used by several dive clubs when they are busy in the summer.

Essentially, you are trading in a wealthy living for the satisfaction of working in some of the best destinations in the world....and that's a happy healthy living.