Sounds like the lush, swampy spot is certainly wet for some reason - if not over your leach field/septic tank, then you are likely right about the "spring".
Check where your water pipe enters house and to its source and make sure this is not a leaking water pipe.
I would check upslope of the spring to make sure is not coming from a neighbors discharge pipe from a sump pump system or pool drain or such. Personally, I would go out with a shovel and dig along near the uphill side but in the wet area to locate any pipe coming into that area, and if you hit water coming up then dig at that point or at wettest spot a couple of feet with a posthole digger or earth auger to look for a pipe or to see if welling up out of ground. Be prepared to fill back in and tamp down if starts flowing enough to cause trouble.
If you have adequate topography to drain away from the house, and probably not into the irrigation ditch (though they might welcome it), then a french drain to lower ground away from the house is probably your answer, assuming you can do so without causing a neighbor problems. If basically flat terrain, then a wetwell (basically manhole with openings at depth) and a pump system might be needed.
First thing I would do is use a hose as a level (google on how to do this) and see if the wet spot is the same level or below the water level in the irrigation ditch, or higher. if level or below could be coming from there through the ground, if higher than not so probably a spring or from a pipe. If at water level in canal or lower, neighborsnot having a problem means nothing - could be a leak near your property only.
Tapping or draining the spring would likely require a water use permit, though in some states less than a certain number of gallons per day is exempt from a permit. Research that before doing anything.
if a spring, cutting the flow off by interception and french drain can work, trying to cut it off by grouting or physical cutoff wall is unlikely to work. The water under the house, if coming through clean and not undermining the foundation, you might choose to live with and just put some bagged pea gravel in the gullies and over the flow areas to stop the erosion and filter out any fines getting washed out to save your sump pump. And maybe a dam of filter-fabric wrapped gravel at the sump to act as a final blockage to erosion and fines washing through. And clean out the sump pump sump periodically so the fines do not build up to the height of the intake and clog or cause severe wear in the pump.
If truly a spring, then waterproofing around the house would not stop it - it would probably still come up under the foundation, so the only fix at that location would be french drains below the foundation level leading away from the house - to lower ground or if necessary to a wet well and sump pump there.
My biggest initial concern would be if the water is eroding the supporting material from under your foundation, or washing out around intermediate posts or columns under your house.
For assistance in determining source of the water and designing a fix, contact a civil engineering company who has a geotechnical engineer on staff, who can evaluate the situation and design a remedy to suit your needs.
If you want to respond back using the Answer This Question button right under your question, and attach a few crawlspace/basement and wet area and overall yard (showing wet spot, house and irrigation ditch) photos using the little yellow icon above the answer box, maybe we can help more.