If the voltage drops just a little below 5.0V, the CPU will lower the frequency automatically to 400-600MHz - it happend to me. Basically you are wasting money on a Pi3 at that point.
The dAISy hat will add it’s own current draw to the Pi3 CAT5 power converter, but not that much.
The main issue will be the heat outside, added to whatever cooks inside that sealed box. it will toast the Pi3 with the hat on.
Why all this complication with PoE and installing electronics outside when AIS (162MHz) travels very well over 50 ft of coax cable?
You can add a CATV amp at the antenna if you really are worried.
I am sending ADS-B (1.1GHz) over the same distance (with a preamp) without issues.