My opinion is that you want to take the boat, that’ll be your quickest option. The FW boat dock is very close to the HDDR building. Taking uber would require taking an internal bus to the front of FW and an uber from there.
Your timeline seems doable to me. Leave FW 6ish, you have until 7:15PM to get to POTC for your FP window, 7:30PM with the 15-minute grace period. Unless you just get really unlucky with boat timing or weather (which of course can happen), doesn’t seem like a terrible plan to me. Yes, you do have to wait for the boat, go through security at the MK dock, make your way to POTC, but that all should be doable in an 1-1.5 hrs under most circumstances.
ETA, an alternative more full-proof method if you’re really worried about it would be to modify your POTC FP back a bit. That should typically be an easier one to modify, under many circumstances.