JP - what you may have are two different problems,
first the water on the driver's side (I Imagine you are talking behind the seat) may be related to a torn liner in the cavity where the top mechanicals are, this liner is made of a flimsy foam and if it gets torn when you wash your car or when it rains water will seep inside the cabin and lodge behind the seat...if unattended it will seep under the seat and damage the control box for the alarm.
To determine if you have a leak in the expansion tank; first check if there's antifreeze smell in the trunk, if so, lift the trunk floor liner right next to the tank location, if the floor s wet and the wood on the floor is spongy then BINGO! a new expansion tank is required...
..VERY IMPORTANT!!! Check also your front cooling fans make sure they are both operating (when AC is on they both turn on simultaneously) usually your tank pops when the system overheats and it does so when your fans do not operate properly...if unattended your water pump will leak and your radiators will crack and leak as well..
...the hot / cold behavior is a reaction of the system when you have some cooling down happening it also happens when you run thru a puddle of water....thermostats are very resilient on these cars, mine has gone over 100k miles in the same thermostat from factory.