My pontoons are taking on water. I don't know how because they are not draining what they are out of the water. Before it used to be just one of them now both of them do that. Any ideas?

Corrosion around any aluminum weld is always suspect. How do you know it's taking on water if nothing comes out?

I take the plug out which is towards the top of the pontoon and get all the water out.

My vote still goes to welds that have corroded and turned porous. That's a possibility even if the boat has live all it's life in fresh water. Get under it with a grinder and wire wheel and go over all your welds. You should not see anything but smooth metal. If you see anything that appears to be porous or grainy looking, you found your leak(s).

I have a 2005 G3 suncatcher 22ft. I have leak on right side ,port, my plugs are on the back of toon 1/2 in up from bottom.
last year i noticed sloshing around water when i puled up to refuel at station, Toon was on trailer where I could hear it. I had this toon 2 years. When I got to ramp and out of way of others I pulled plug. I drained out 60 gallons of water, i had 5 gallon bucket to use for measure. I have searched allover ,under, around the 2 toons, I put a valve( like on tire )
so I could put air in it, ONLY put in about 3 pounds pressure, I found one leak up front where the "splash guard" is welded on. I fixed it, but still will get about 5 gallons on a Saturday -Sunday outing, I called G3, they said do exactly what I did, So I still take on some water.

It's hard to wrap around a boat builder telling you it's normal for their boat to take on water..

Im with bob on this one, I cant imagine a pontoon taking on water and that being normal.

Why would the water get in but not leak out when it's not in the water?

if it leak is not at the lowest point on the pontoon, it will not come out on its own, gravity I guess