** edit to remove comments-wizard***
Marmaris is a great place and the people are fantastic,friendly and warm,to suggest you'd be spat at and sworn at for not going in their shops is not something i'd take any notice of AT ALL and i'd love to know where this is meant to happen,following a trip every 6-8 weeks for the last 8 summers we've made many friends over there('we' includes greeks,who the turks are said to hate,no evidence of that in marmaris at all) and reading this would hurt them.
the trips are 50 euros and that almost always includes collection from your resort and return transport to the port in rhodes,you'll need to surrender your passports the night before you go for the immigration people but it's a simple process,the boat across passes through some amazing scenery on it's way into marmaris bay,through narrow inlets and past remote villas on clifftops,if you do go then the best way into the city is in one of the yellow taxis that park up at the ferry terminal entrance,much quicker than the bus that comes with the ferry ticket as this takes you on a 'tour' that's really just a long winded drive,the taxis take 5 minutes and cost around 7 euros,just get in and ask for the bazaar area,if something more authentic is your thing then ask how much it is to go to datca,a smaller town on the same peninsula.
Back on topic,the best way to the waterpark from pefkos would be to get the rhodes town bus and get off in faliraki(aim for the stop just after the top of bar street opposite bubbles pool bar/isis travel)walk 400m down bar street to the square by mcdonalds and get the trainaki(tourist train) or look for the blue liveried bus that takes the coast road to rhodes via kalithea starting on faliraki club street,sounds complicated but you'll see how simple it is once you get off the first bus. the waterpark is opposite the esperides beach hotel and is MORE than worth the trip,they claim it's the biggest in europe and i would probably concur,it's an excellent day out.
Other than that i'd recommend ANY trip that takes in the embona/siana area of the mountains,or if you want to hire a car then do the usuals of seven springs,embona,the mountain passes,prasonissi beach etc etc etc,doable in one or two days.or simply get up to rhodes town and see the old town,or if in a car go up monte smith to the acropolis/old stadium and ampitheatre site,and have lunch in one of the small pavements cafes along mandraki harbour where all the day trip boats moor.
If you're feeling brave go to the taverna at the southern end of pefkos beach and do the mental trip out on their aeroplane,they take you out over pefkos,out to sea,up over lindos and take in all the sights there like the lion's head,the acropolis,the 'beast' diving cliffs etc and then fly you back at low level over pefkos,it's pretty exciting stuff as long as you havent just eaten a big meal!