That is a really difficult situation. The best option would be to improve the lighting itself, but you are not allowed to use a flash or even place lights.

• The shutter time needs to be fairly short to freeze the motion of the player and the water, as well as to avoid camera shake visible as blur.
• You can use your lens aperture wide open, at the cost of depth of field

• The easiest option is sensitivity. Crank the ISO up to a level that doesn't give more noise than acceptable for you. For the Nikon D90 that might be 1600 or 3200.

• If you have a fast (and expensive) tele lens, you can get your framing from far away. A monopod (or tripod) is advisable for longer lenses (let's say beyond 100 mm) to minimize camera shake.

• If you can get really close to the pool, you can chose a standard lens or wide lens. That gives you wider apertures, less risk of visible camera shake, plus interesting perspectives.

It is not possible to give you a list of good settings, because it depends too much on the scenery and the amount of light available. If possible, please upload an example image for review, discussion and advice.

Finally you should consider if your beloved D90 is the right camera for this purpose. Cameras and their useful range of ISO have been improved a lot in the past 10 years. You might want to lend or rent a recent full frame DSLR and see what it can do in low light situations.