The biggest point that seemed missing to me: the author talked so much about increased evaporation, but isn't water vapor one of the worst greenhouse gases? Sure you mitigate sea level rise, but at the cost of increasing temperatures? That doesn't seem like a great tradeoff.
You are correct that water vapor is the primary greenhouse gas. Despite that, water vapor emissions are not generally considered to impact warming. I think that is because the water content of the atmosphere is already in balance due to the massive amount of evaporation and condensation (e.g. rain) that is constantly going on. If we dump some extra water vapor into the atmosphere, that will balance out as a bit of extra rain and/or a bit less evaporation somewhere else; atmospheric water vapor levels won't change.
I don't know whether a large scale, ongoing intervention, such as permanently flooding a large area, could nudge this balance sufficiently to result in a sustained (if small) increase in atmospheric water vapor content, and thus an increase in warming. An interesting question!
I guess it depends on whether such canals increase the net surface area. If you raise the ocean by 3mm across the globe, does it result in the same net increase in surface area as flooding a large swath of the Sahara?
The biggest point that seemed missing to me: the author talked so much about increased evaporation, but isn't water vapor one of the worst greenhouse gases? Sure you mitigate sea level rise, but at the cost of increasing temperatures? That doesn't seem like a great tradeoff.