Both Blackpool and Ronaldsway are designated airports for the PTA.
Thus the requirements of the PTA are met by landing at Blackpool prior to leaving for Ronaldsway. If the Special Branch haven't bothered to man their patch then there should either be a telephone provided or some form of letter box system as previously mentioned.
The 24hr advance notification request only applies to non-designated airports.
The Special Branch in the IOM are a friendly lot and once you have been through once, they keep you on file which speeds up your next visit.
The list of designated airports is in the AIP at GEN 1-2-16
Personally, I would consider a life raft essential unless using two engines. Survival time in the water at the coldest time of the year (which is now), is only a matter of minutes.....not even long enough for the SAR aircraft to get airbourne.
Regards,
DFC