Keep in mind that any agreement wrt your land with a conservation commitment will reduce its potential sales price because it will reduce the # of potential buyers. Most land buyers are not interested in land with other peoples restrictions on it. You may also be assuming a potential liability for other people.

Also if they are providing half the money and the planning advice - then for who's purpose are they designing the pond. This comment is applicable to lands in the US. I can't comment on land ownership in Canada.