Ground looking radar is a possibility; I’ve used it on construction projects to determine depth and location of complicate underground utilities. We did a one meter grid on one project and then transferred the paper print out, which looked like a slice through the ground showing cross section of pipes concrete pipe conduit under lying ledge. That was many years ago modern equipment outputs directly to CAD
What are the lake bottom conditions hard or soft. If it is hard bottom then good sonar is likely to be the fastest and cheapest.
Also you might want to check the web for the guys who have salvaged tanks in Eastern Europe several tanks have been pulled up in the last few years.
http://www.mil.hiiumaa.ee/2000_09_14...-36/index.html
Also look up finding the Monitor project of US Navy, US Naval Academy and Duke University combination of sonar and ASW equipment. Do you know any Naval reserve types that could arrange a training flight over the lake.
Bradford Washburn, did a detailed mapping of Squam Lake in New Hampshire write – up of that project might be of help.
There is a D-9 Cat at the bottom of a small lake near here which supposedly sank into the soft peat bottom of the lake without leaving a mark.