In reply to Flinticus:
Its letting water in where there is constant flexing. The membrane could be knackered.
First I'd try filling then up with warm soapy water for afew hours. This will clean the membrane if it has one. You will amazed at the dirt and grime that comes out.
And then use a spray on water proofer.
In what way are you coming into contact with water so much its letting it in? Wouldn't think the odd puddle or stream crossing would be enough to let water in. And I wouldn't have thought rain would manage to build up enough to do the same.
Having a good DWR is the main barrier against water, stopping the fabris becoming saturated.
Clean your boots after every use, or once weekly if using everyday