I have lived in Monmouth Beach for over 30 years. This is an awesome family friendly community, surrounded by the Shresbury River and the Atlantic Ocean. Please ignore the comments about Mosquitos. We don't have any more than Dorothy in Kansas. Flooding is an issue but it is something you learn to deal with. The ocean usually isnt a threat...only in a hurricane. Hurricane Irene did more damage in surrounding towns than here.
I am the broker/owner of the GANNON HOLSEY GROUP in Monmouth Beach. Let my experience, knowledge and negotiating skills work for you!!
As stated a large concern would be mosquitos and environmental easements through your property. I have clients with an entire acre that they cannot use because there is a drainage easement which now holds standing water and there is a new conern that it may be a habitat for box turtle, even though it was never supossed to HOLD water. My clients cannot build a barn for their horses now do to this and the neighbors are fighting for a correction because the mosquitos have become unbearable. Best of luck and talk to the neighbors!
Mosquitos. Flooding may also be possible. Flood zone maps are available, probably either from the town, Township, or County. Mosquito maps are not. Ask locals (neighbors are best) about these issues and determine if they are enough to concern you.
I think your biggest dangers are mosquito bites and storm damage... but the storm damage is a risk you need to be willing to live with if you live at the beach!
More seriously, figure out the nearest coffee shop or diner nearest the potential purchase, and go and ask folks how they like living there. Most folks are happy to give you feedback if you ask in a non-threatening environment.
Its not the marshland you need to worry about as much as the flooding. Monmouth Beach is a great town but much of it is prone to flooding when the river rises during heavy storms. I would try to stay away from homes within 2 blocks of the river. Other than flooding, marshes do have critters (rodents) but that's really not usually an issue.