SIU have different contracts for different companies. Oftentimes they have different contracts for single ships within the same company. The pay scale can vary a lot from company to company or between ships owned/operated by the same company. Also there are a lot of other factors you have to take into consideration that can affect your earnings ie vacation pay, benefits ( SIU have 3 different plans) and unemployment benefits that you can collect on your time off ( that depends on which state the company is registered in).
To give you an example. Some ABs on some ships make less than OSs on others.
As an electrician, working for Maersk for 4 month, I made a little over what I made working for Horizon Line in 2 month. Then again Maersk have a lot of contracts and different pay scales. If you tell me what position you looking for I can give you an average.