>>It seems 6063 tubing is the choice for most - any recommendations for other tubing
>>that would be even more
>>corrosion resistant that would work well with skin on frame construction?

6000-series aluminum is softer than 7000-series, and easier to work with. Aluminum is corrosion-resistant, not corrosion-proof, but that is true for almost any material other than plastic - wood rots as well.
Aluminum works very well in SOF construction - the fabric wraps nicely around the tubes.

>>If going the aluminum with plywood route - would I need a drill press for the recesses for the >>gunwales/stringers/etc? - Like for HPDE example on yost werks site (though not the snap in part, just a half circle)?

The cutouts in the plywood frames should be circular to conform to the shape of the tubing. You can easily cut them out with a jigsaw. You'd need to flatten the last several inches of each aluminum tube with a hammer - make sure they are flattened on exactly the same plane. Then plug then ends with some kind of silicone (similar to what is used in bathrooms).
Other than that, no other fabrication of aluminum would be necessary. The tubes don't need to "snap" into the frames the way HPDE does, because you are not making a folding kayak where the tubes need to be re-attachable to the frame.

>>Assuming my only choice to secure the tubing would be lashing?
Certainly the easiest, you could also try drilling holes through the tubing and screw the stringers to frames if you like. However, the joint would not be as strong as with lashing. If lashing, choose low-profile artificial sinew so it does not protrude much from the tube. If you really want to avoid it, you could try making a small depression in the spot on the tube where the lashing fits.