We really didn't get wet jetting in the pipes once we figured out how to do it. We just had a well pump that we used for the underground sprinkler system, and adapted the outlet to garden hose thread. Ditto for the top of the pipe (we had a manual pipe threader too). At first we tried tape and rags, or just holding the hose in the pipe with rags and that didn't work.
We installed a 75' seawall there too. We used the interlocking "W" shaped steel, either 5/16 or 3/8" thick. We washed/jetted that down too, using a piece of 1/2" or 3/4" pipe on the end of a hose. We used 8' lengths of the seawall steel, going 4'-5' down into the lake bottom because it was holding back soil. We capped it with a piece of 4"x4"x1/4" steel angle. That was installed around 1980. Still holding strong.
Now they have rubber reduction collars that are held in place with hose clamps, that might work without having to thread the pipe.
I doubt that you will be able to get enough force on the 6x6's manually to get them far enough into the bottom of the pond. To prove that to yourself, try digging a hole at the edge of the pond in a few inches of water, then try to pound a 6x6 down into the soil.
We went by the rule of thumb to have at least 1/3 of the length of pipe that stuck up out of the bottom of the pond into the bottom of the lake, going up to at least 1/2 the length (or more) when installing in shallow water.
We also found out that by installing a martin house on a pole that was attached to one of the pipes that was holding up one corner of the boat hoist was a bad idea. In one year, due to either the birds landing/taking off or the wind blowing on the martin house, combined with the down pressure of the boat that post sunk 3" over the course of a year. Our "fix" was to support the horizontal part of the boat hoist (floor jack with a piece of wood on the ice), unscrew the large pipe we slipped over the pipe we had jetted into the bottom of the lake, set a 3" long piece of pipe on top of that pipe, and re-attach the slipped over piece of pipe to the horizontal part of the hoist. AND put the martin house on it's own pipe........