You are the perfect candidate for a VHF radio operating on MURS frequencies. MURS is a set of five frequencies set aside for license free operation for business or personal use. The VHF frequencies will afford you much further range outdoors than with traditional FRS/GMRS radios because of the propogation of VHF signals-- they tend to bend around trees, foliage, and mountains better than UHF. Power limit is 2 watts but on flat ground and water that should actually give you the 4-5 miles you are looking for.
Recommendation: RDM2020 or RDM2080D by Motorola. The common PTT switch problem has been fixed and they are fairly durable radios for what you will be using them for. 2nd choice would be another commercial portable like HYt TC-610P (waterproof!) programmed to MURS frequencies.
I think you are asking if the MURS frequencies are relatively free from interference or other users?
MURS has been almost dead up until a few years ago. That is here in northern NJ, there were few if any users operating on MURS. It has become increasingly popular due to Motorola programming the first two MURS frequencies by default into their XTN series radios and now offering the RDM series, a dedicated MURS-only RDX series radio. MURS is still relatively quiet around me and free from other users, static, etc. stay clear of 154.600 and 154.5700 and use the 151.xxx freqs and you should be fine. You will find A LOT of garden centers and summer camps using the 154.xxx freqs when the weather gets nice.