Park at the obvious parking area by the put-in on Grafton Pond Road, just past the dam. If it is a weekend and the weather is good, you may find that the lot is full unless you get there early, but you can always park along the side of the road somewhere not too far away after unloading your watercraft near the put-in.
The most direct route to the cache is straight across the lake to the far (northeast) end. Toward what appears to be the end of the lake, you will find a tiny island which is composed largely of a big rock that is a popular place for visitors to stop and swim. Pass by that island on the right and you'll find a narrow, shallow waterway that continues on into a marshy area with croaking bullfrogs, dragonflies, and marsh flowers. You won't have to go much further to reach a place to go ashore near ground zero (see the Reference Point coordinates). From here it's a short bushwhack through the woods. The woodsy part can probably be done with shorts and sandals but bring shoes and long pants if you want to be sure to avoid scrapes and scratches.
Note that this cache can actually be accessed by land, though it would likely be a difficult bushwhack. Also note that gas-powered motorboats are not permitted.
Nature is calling!