With vents full open on bottom and top your fire is burning HOT HOT HOT because it is being fed too much air. The water in the pan is keeping the chamber temp down because it is absorbing the heat and boiling off. You are using more WAY more fuel per unit of time than you need to maintain your temps, and your coals are ashing up quickly causing the temp drop. Its a physics thing.
Take off the lid, open the side door, and with long tongs or a stick "gently" stir your coals to knock some of the ash off your briquettes. You take the lid off so you don't ash up your meat, plus the influx of air sucked through your bottom vents will keep the fire burning hotter for chamber temp recovery for the fuel you have remaining. You should have PLENTY of fuel left to finish your smoke without adding more.
After doing the above close your bottom vents down to 1/4 to 1/2 open. Do the same with your top vent, starting with 1/2 open. Let the smoker temp stabilize. If the chamber temp looks like it is going to shoot past your target chamber temp, close the top vent down to a smaller opening to quickly reduce the air feeding your fire.
I'm all about air management when I use my
WSM. When I fire up my 22.5"
WSM I have all the vents full open. After I load my
WSM with charcoal and wood I never use a full chimney of hot briquettes. If I'm doing a 225F smoke I use 1/4 to 1/3rd of a hot chimney, about 1/2 chimney for 250-275F smoke, and 3/4 chimney for a hot 300F+ smoke. I let it burn like that for about 5 minutes before I add the center section, water, and lid. As soon as I assemble the smoker I close down the bottom and top vents to the settings I want for the temps I want. I let the smoke get to the stage is it is starting to get lighter then load my meat. My
WSM likes to cruise about 260-270, but it is leaky. If I'm going for a 225F smoke I'll often use only one vent on the bottom for temp control, closing the other two down completely. I match the top and bottom vents.
Air-Fuel-Heat is the fire triangle. Air has the most immediate impact on a fire temp so that's what I use to control my WSM. Try a dry smoke (no water in the water pan) some time with chicken to truly understand what I wrote above about managing temps with air flow instead of water. You'll use much less charcoal once you understand.
