IRS Taxation confusion for Australian Non resident

[Kyfho]

Good afternoon all,

A somewhat complex taxation situation has developed and I would appreciate initial guidance for a pending IRS W2 submission.
My situation is thus;
I work for a largish multinational deepwater offshore energy service provider with an office in Houston, Texas.
I'm an Australian national though not for taxation purposes as my abode is a yacht - worldwide.
Working within the Gulf of Mexico averaging in excess of 185 days/year (US region of economic influence) utilizing the USA issued OCS B1 visa.
Payment for my services is from the company's Singapore office.
The visa conditions stipulate that I transit the USA to join/depart the vessel. If wishing to visit the USA I am required to depart - closing out the I-94 and re-enter under an existing Visa waiver aggreement.
Currently the company is withholding FIT, Medicare and Social Security taxes from my pay for the IRS, for which I do NOT receive benefits, health care, pension etc.
What options are available for my situation? (Excepting the obvious work else where option as I'm rather attached to the current vessel/work)
Assistance please?
A Houston based CPA have similar dealings for a follow up?

Regards, Callum.