some questions

1) is there a standard salary per year? and the daily rates per project are additional to this salary? if yes, how much is this salary about?

No. It varies significantly by area of the world, the country of origin of the company you are working for and your nationality.

2) are the money earned offshore subject to tax?
Generally yes. There are very few LEGAL ways of not paying any tax. Again, each country has its own tax laws. Here in the UK if you work onboard ships and spend > 6 months a year outside the UK you may be able to claim your tax back.

3) how many days do projects run? how many projects are per year?
Between 1 day to 365 days. This is an impossible question to answer, there are so many areas in the world I doubt anyone has counted them.

4) is there an age after which you are too old to enter this industry?
Yes, when you can no longer pass your offshore survival or medical.

5) what qualifications/experience would be most beneficial to get a ROV job as a trainee?
Hands on technical qualifications and experience. Companies do not take on trainees who already have experience in ROVs. They take on technically qualified and experienced people who can repair and maintain the equipment. Learning how to operate the ROVs begins ONCE you have the job.

Some Training Schools take on butchers, bakers and taxi drivers and charge them a fortune for a bit of paper which counts for very little unless you have the above suitable technical background.

HNC level is appropriate, Degrees are less useful – you do not learn to solder with a Degree for example. We are generally there to repair and maintain, not design ROVs.