Map making project help!

[Zeal]

I fell in love with a lake that is 5 minutes away from my house, sadly, there is not even a contour map for it.

The lake is an absolute jungle: thick bur weed and cabomba cover the bottom (which I believe is 99% mud) and reach the top, lily pads grew nearly everywhere (which was unheard of) and as a result, you cannot see on the depthfinder anything other than a mass of green.

I'd figure that I would have to take my electronics with my yak when the weeds recede and die off to really get going on this but I have no idea how to start.

Ideally, I want the end result to be a nice topographic map but right now, a contour map would be a nice thing.

Anyone have suggestions or knowledge in how to do this sort of thing?