Williams has NCAA D3 swimming, but club water polo. Pomona-Pitzer (the two schools field joint teams) is NCAA D3 for both sports.
Williams appears to have a strong swimming program; women’s swimming was ranked #4 nationally in NCAA D3 last year. Pomona-Pitzer finished just outside the top 25.
http://www.ncaa.com/rankings/swimming-women/d3
There are relatively few NCAA water polo teams, so the NCAA treats it as an interdivisional sport, where schools of all levels compete together. So there is only one NCAA women’s water polo tournament, not separate tournaments for D1, D2, and D3.
Last year, Pomona-Pitzer made the NCAA tournament – and since there is no D3 tournament, it was the same tournament that schools like USC, UCLA, Michigan, and eventual champion Stanford played in. OK, Pomon-Pitzer didn’t advance past the first round, but it’s still cool. Williams is ineligible to participate in the tournament, since they don’t have NCAA water polo (neither do any of the other NESCACs).
http://www.ncaa.com/interactive-bracket/waterpolo-women/nc