I have read a few posts via google and not sure if they match mine so I thought I'd post my own.
My 2002 blazer LS has water only one the drivers floor board, its not wet behind the pedals just on the floor in about a 1.5 square foot area in front of the drivers seat. I can now hear water sloshing around when I turn slow.
I have taken the cowl off and flushed there and was very little debree at all anyhow. I also have full heat, no issues there that I can tell.
Any ideas would be great. Because of the heat being fine Im finding it hard to think its the heater core.

I also need a spring on my hood latch cable but that can wait for later, I use the shimmy method.

 

Were you to have antifreeze in the cooling system with the heat on you would have a sweet smell permeating the the cab. If you just had water you would not smell this. This is how most people notice a bad heater core.

 

But wouldn't the water be running down and not just on the floorboard. As far as I can tell its not running down. And does that mean I have to replace it. I read somewhere about a fuse reset or something about that. Could that fix it?

 

check your door seals

 

I have looked in that area, they seem to be pretty well dry towards the inside of the vehicle. I live in Portland so we get lots of rain. Any good methods for checking them?

 

Please note the little drain tubes on either side of the unit-plainly visible:these are clogged with the common debris of your area-tree issues in the northwest are usually the cause of this! I had this happen to a 78 GMC truck I had up there in Granite Falls-but it didn't have the tubes! It clogged the door weatherstriping! Gotta love those pines!
The rain only made it worse! Best of luck!

 

check underneith and make sure you dont have any rust holes

 

do you park on a hill? if so could be rear door seals, that's what mine did on the passenger side, parked on hill on a downslope and the water pooled on the passenger floor.....when you hear the water sloshing does it come from the rear of the truck? if it is coming from the rear, pull these plugs, one on each side, should have quite the shower coming out, clean and replace.

 

the glue may have come undone in one spot on your door seal, that what happened to mine. just a big enough spot to let water through, then it runs down and comes out above the speaker. to test it you could have a buddy spray water on the top where the door seals while you sit inside and watch for leaks.

 

you sir just solved my problem. anyone that rode in back would complain about a water sloshing noise. went out and just took a nice blazer supplied dirty water shower. haha. though this is not the original posters problem solved...