A lot of lochs and river beats have limitations due to leasing of rites to local clubs or are owned by estates. However bank fishing usually costs little and I've found that when asking for permits the fee has often been waived. Do some research for the area rather than fall foul. My understanding, and I'm open to be challenged, is that if found fishing illegally i.e. poaching, by the gamekeeper he can impound all your belongings including a vehicle if that is believed to have been supportive of you poaching. And then they can prosecute you.
Whilst you have right of access to land it is only on foot and only whilst you are on passage. It doesn't lend itself to fishing, snaring, hunting etc. Asking and paying a small amount would likely pay dividend in guidance of best tackle or best locale. Equally it may help develop a trusting relationship which may give you greater access. Good luck.