I don't think you need a megadose Polly, as you are swimming outside regularly and getting natural vitamin D from the sun (but don't use sunscreen). I know George Jelinek gets his vitamin D this way even on cloudier days, by swimming. I think you should increase your supplements though, to 5000IU/day over winter to get up to 200nmol/L or 80ng/mL, then get tested again in the spring. You may find that lots of swimming outside is enough to maintain this better level as the vitamin D scale is not linear ie it takes a lot to raise your level, but less to maintain it for most people.
There is an explanation here about exposure to sunlight - you make 10,000 -15,000IU a day from sun exposure, even through water.http://www.overcomingmultiplesclerosis. ... Vitamin-D/
I agree that calcium is not an issue, this is from the Diet FAQs:Where do I get calcium from if I exclude dairy from my diet? Will I get osteoporosis?There is abundant calcium in this diet. The real problem with calcium in western societies is lack of sunlight. Sunlight results in vitamin D being produced in the body, and vitamin D's main job is to absorb calcium from the diet. A whole calcium supplementation industry has grown in western countries as a result of sun avoidance. Foods such as dairy are heavily promoted as being healthy because they contain calcium, ignoring the real health problems they may cause. With adequate vitamin D, from sun or supplements, there is no problem with inadequate calcium. Osteoporosis is unlikely too for people who get enough vitamin D.