Bora Bora with hotel transfer?

I am staying in Bora Bora at the St Regis for 2 nights and Westin for 2 nights from 11 December.

Unfortunately, I am on a budget and I would like to avoid excessive hotel transfer charges. Has anyone got any recommendations or experience getting to St Regis without using their transfer?
I understand a boat to the mainland and then back might be a cheaper option, I assume the same could be done to get from St Regis to Westin.

Thanks,