There aren't any venues on lake michigan in Fennville. if you're set on that area you'd have to rent a house and do an outdoor tented reception - availability will depend on number of guests anything over 75 will be problematic for a house rental.
So hard to give you good suggestions without really knowing all your requirements but what I can tell you is that rentals that allow weddings are hard to come by especially with a Fennville address, there isn't much that actually sits on the lakefront in that particular town either.
Start with the local vacation rental companies (Lakeshore Lodging, Mill Pond, Shores). They won't put "we allow weddings" on the online listing so you have to call the office and actually speak to someone to get that info; but at least look at what's there before you call because some will agree to contact a particular home owners to get permission for a special event...just be prepared to rent the house for a full week during high-season.
If you can't find one there that is available for your date (or price range)go to VBRO, find a house and ask the owners directly.
Either way, take out a one day event rider from your insurance company - some rental properties will require it, but I say better safe than sued - LOL!
If all else fails, there are a lot of other really nice beach towns that surround Fennville, that might have just what you're looking for...Saugatuck, Douglas, Holland, South Haven, Glenn all have rental homes and/or venues on the lakefront (depending which lake you want).