If they float over it, no. The lake is a public right of way.
If they use a pole, anchor or get out and push they might be trespassing if your land is posted. I've never heard anything about NH law on submerged lands so I am not sure about that.
There are some exchanges on this thread about rafting that you might find relevant even though they are another side argument...
http://www.winnipesaukee.com/forums/...highlight=feet
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