i think its an interesting idea - lets have a look at what this change might do:
make navy a bit more static - interestingly the difference here is in consistency - you wont go from winning position to losing position in 3 seconds.
this will also stop people in a winning navy position to snowball as much by nuking enemy navy just out of smd range
cost for this would be 0 shields on your expensive asf smd that takes 999 years to build + load anyway, so it could very well be useless.
so i guess it would be better for uef as usual and worst for cybran - since already cyb navy is weakest at late t3 and uef is best, this would tip it even more in their favour.
and it also hinges very heavily on shields (shield boats in this case) so its not easily balance-able - you would need to rebalance navy as a whole to get this change to work, or face an unnukeable uef and have no chance as cybran with 0 shields to defend your 3000hp smd vs the uef battleship with 54938493 range (or anything else for that matter).
of course you could go one step further and make shields also amphibious, but then you run into the problem of navy becoming even more static and basically an arty war. also harms creep + shields is something you might want to check up on, but it would even up the playing field at least, still i would think this is not in a good way - since cybran is the only shieldless faction you could make this only for cybran, which is an almost viable choice, but still leaves a little to be desired.
alternatively you could make cybran smd like harms - underwater which would even things out a bit but torpedo bomber snipes are a thing so not sure if that will really help.
in short, sounds like a nice idea at first, might not be so nice in the end
hope this helps