How can I build a seawall?

That's a tall order, but it can be done. See if these get you started:

https://www.doityourself.com/stry/building-a-seawall

https://m.wikihow.com/Build-a-Seawall

https://extremehowto.com/keeping-the-water-at-bay-build-a-sea-wall/

https://www.themalibucrew.com/index.php?/forums/topic/56092-stairs-into-lake-traction-issues/

https://www.pinterest.com/kingjohnj/lake-stairs/

You want to build a sea wall by yourself?

I can’t physically build a real sea wall by myself and I’ve been in the architecture & construction industry for 30 years.

Sea walls are to stop erosion, is that what your goal is?

Lakes have flood plains.

If you don’t own the entire lake, there’s likley jurisdictional agencies (City, county, HOA, Corps Of Engineers) that will have Rules about sea walls, docks and more.

Every kind of lasting sea wall design i could specify requires at least a tractor-driven auger and dump truck access or concrete pumper truck to the site and some heavy duty tools.

If you want to build a retaining wall, outside of the floodplain, with those stacking/notched concrete blocks, you’ll have to trench and pour a raised concrete foundation with some sort of system to hold the bottom course onto the foundation.

The backside will have to be back filled with a French Drain System and gravel.