Production or drilling?
drilling rigs drill the wells. Production platforms produce the oil. They need mechanics, operators, electricians and so on. I wouldn't think it's impossible to get on if you have one of those skills.
No, you can't spearfish. I work for an oil company and worked on a platform for 12 years.
two week rotations are common. Life is ok and you get used to it. Be prepared to miss everything at home. Also, be prepared prepared for every home emergency to happen the day you leave.
you can make 100k pretty easy with the right skills. I interviewed a 27 year old electrician that worked on a drill ship 15 years ago. He asked about the salary. He said he made 172k a year and he was wondering how it matched up. So yea, you can make good money.
I expect the industry will still be fine in 4-8 years. I sure hope so because I retire in 6.