For a basic rowing boat hire service, fishermen will give you custom all year round, while tourists will offer lucrative trade during the summer.
You might want to give thrill seekers a high speed run out at sea, allowing them to experience the excitement of the ocean in safety and (comparative) dryness.
Or, you could provide leisurely trips along scenic waterways - many people never have the chance to drift along a peaceful river, relaxing in the gentle tranquility of a quiet summer's evening (you could use that as an advert!).
When setting your price you'll need to cover any fuel bills and other expenses and add your profit margin on top. Look at what similar services in your area (if there are any) are charging.
For a trip in a small motor boat you can charge around £30 an hour, whereas £5-£10 per hour is about right for a small rowing boat. Anything up to £200 per family is the going rate for a weekend trip or break.
Advertise this venture in riverside pubs, fishing shops, on the beach and in the local paper. Make up flyers and hand them out to passers by, and put up at least one attractive sign close to where the trips start.
Contact your local council regarding obtaining the necessary permits. You'll require public liability insurance, and your vessel will need to be insured too.