As we have already worked out the trout are to be found on the windward bank. And generally close in too. Often in April the only viable approach is to anchor nose down and fish deep. However given two circumstances, 1 - favourable wind conditions, 2 - free space to fish large amounts of shallows, it is possible to drift fish.
Try drifting with intermediate lines and pulling wets or nymphs. Alternatively, if over deeper water, or in windier conditions, a HiD and nymphs/wets. Occasionally the only thing to be done is to throw a HiD with a lure over the back of a drifting boat and aim to get deep (20 odd feet).
Alternatively fish Northampton Style; lead line over the back with big lures and aim to catch a big brown. There exists great subtlety in this method, fly action, angle of fly descent, angle of retrieve and speed of drift all become significant. If you want to attempt to catch large browns them this is the method for you. I am going to make no attempt to cover this technique, as it is somewhat specialist, however I would refer you to Steve Parton's book in the appendix.
Anchor fishing is in many respects as for bank fishing techniques, however you may find that you have to fish one line deeper.
Expect fish to follow you right up to the side of the boat. Hang the flies before you recast. Above all fish slowly and methodically. Once boredom sets in and you start to fish on autopilot then you may as well pack up and go home. Change depths and move frequently.
Eventually you will locate fish. Once found, fish hard and stick with them until they stop feeding. If you and your boat partner fish together in an organised fashion it is possible to fish each anchor position thoroughly in about twenty minutes. If you catch sod all then move. And keep moving. It will warm you up, keep you optimistic and catch fish.
Boat fishing in April is hard work. The wind gets through to you, there is no shelter, it's cold and often just when you think if couldn't possibly get any worse, it rains.
Also you have to be a bit of a nut to constantly fish boats at this time of year, bank fishing beats boats by three fish to one. But you might just learn something. At least that's what I have been telling myself for years, and I am still not sure what it is I have learned. But I am sure it's something worthwhile knowing.