When buying a business, the question to ask is simple.....
What am I going to pay for the business and what could I do with that money on my own if I decided to buy my own equipment/trailer and advertise?
I don't think anyone can really tell you if this is a good deal or not - the 10 accounts for $700 a month in revenue is basically $70 per house per month. what is included in this $70? Mowing? How many times? other work?
If you were able to sell your boat for $2,400 and you have $600 cash I assume you could buy a trailer, used mower and hand held equipment. Then the $500 a month you were going to pay him could be put towards advertising.
This is just an alternative scenario.
I am not telling you what to do, just think it all the way through is my advice.