The two major swims you'd be interested in are the 'Round Jersey' (RJ) and the 'Jersey to France' (J2F) outings.
Tidally assisted the RJ is very roughly 41 miles. Swum on 'springs' it would take a good swimmer some 10 -11 hours. Far different from a Channel swim should you be a bi-lateral or one who breaths to the left then it's a scenic swim. Breathers to the right only have the view a-head and whenever they feed to catch up with the castles, cliff's and bays. At times along the north coast we occasionally log swimmers at close on nine knots, but I hasten to add only for short periods, but it's all good fun. The waters are clean and fresh in relation to the Channel. The tide is never against you, unless I've got my navigation totally wrong. At the low water mark, normally in the area of Corbiere Lighthouse there is little or no assistance and after eight hours you have an hour or so working hard, however by St. Brelades Bay the tide is building once more. RJ swims start about two hours before high water and finish at the southerly end of Elizabeth Castle breakwater on the opposite side of the wall to the start.