From the query it can be make out that the ships happen to be in Indian territorial waters.
If so, section 9 of IGST shall apply and POS shall be as per this section.
Section 9
Notwithstanding anything contained in this Act,––
(a) where the location of the supplier is in the territorial waters, the location of such supplier; or
(b) where the place of supply is in the territorial waters, the place of supply,
shall, for the purposes of this Act, be deemed to be in the coastal State or Union territory where the nearest point of the appropriate baseline is located.
Section 9 overrides the entire Act as far as POS is to be ascertained.
For ascertaining the POS you need to determine the distance between the ship in water to the nearest state and clients location and ascertain the C+S or I accordingly.
Though payment is received in USD, it shall not be export.
Thanks and Regards
CA Susheel Gupta
9811004443