Depends on what time the fireworks get over. The small boat (CR/WL/FW) runs 7 am - 10pm. If the FW's ended around 9:30, you could certainly walk over to CR and take the small boat to WL.
Another idea, take the large ferry to the TTC, then the bus to WL.
During our 2003 Sept trip, Spectro & FW's were only shown on Sat nights. It was MOBBED, as you can imagine. We walked around about 10-15 minutes before leaving MK. When we went out, lines were extremely full for everything. Even the monorail had lines all the way to the bottom of the ramp. We got in the very long WL boat line. From that point to the time we walked into our room, it was 30 minutes. I timed it. The boats were coming pretty fast.
Enjoy your trip!