I would really try and get the neighbors on board before offering to fill their part of the pond in.While you might not get them to share the costs at first it is in their best interest as a owner to have a viable managed pond vs a mosquito pit that detracts from the property value.
Speaking of value,it might be wise to talk to a few realtors to see how the property values would increase with a proper pond in place vs a neglected mosquito hole full of algae.Chances are the comps around you without a pond have a big difference in price.I think if you can show that what you would like to do will increase the value of their properties too you'll have a much better chance to move forwards.
I'd also think about speaking to a real estate attorney about the possibility of forming a pond owners association of sorts in the form of a HOA.While it is a long shot I know of one place here in NJ where a few owners formed a HOA to improve a sub division.While most of the owners didnt pay for the initial improvements they did agree to pay HOA fees,some of which went to repay part of the initial investment and the rest was for maintenance along with future costs.
While it is a long shot that they will share the initial rehab costs,by working with them to increase their property value you'll hopefully get a green light to do some work and possibly share management costs in the future.
