You will probably need a 3" trash pump to keep up with the ground water. I dug my ground water pond wet without pumping much but pumping would be easier digging. I would start in the middle and keep working outwards. Let the dirt sit for a day and dry a bit if you have to for hauling.
If you dig it wet there is no way you will be able to get it 15 feet deep. You will have to be careful with the suction end of your hose so it doesn't fill with sand. I would recommend tying it to a tire or inner tube to keep it off the bottom. Be prepared for challenges digging with that much water present. My pond stays a fairly constant depth all year around but we have had fairly wet years since I dug it 3 years ago. Try a test hole early to see where your water level might sit.
