Hopefully this may will be better weather than last may, which was a disaster with abnormally strong winds for most of the month.
In may the best of the fishing is never too far away from the shore, and wading can be deadly. Boats are not essential, but are handy for covering alot of ground and fishing the edges.
If you want loads of fish, then Harray and hundland are the top choices, ive never really fished boardhouse that early on but it might be worth a go. In swanney you always have the chance of a good basket and maybe a 3lber in the net too, but you could just as easily blank, so its a flip of the coin!
Midge is the main food on the menu in may, so black flies are generally hard to beat.
Coch zulu, kate mclaren, bibio's etc etc. Funny enough peach can sometimes be deadly when there on midge, muddlers or hedgehogs, i usually have one of the two on my cast. Small flies fished unusually well last year, 12's and very often 14's takeing the best baskets and the biggest fish!
If the weathers good, dont forget to have a wade on Skail or clumly, there both trophy waters and may's the time to fish them, probably with bigger flies though, atleast on skail, ie soldier palmer in clumly, peach tailed wickhams muddler, petersons pennel, shuggies in skail.
Mind and have a look here
Orkney Trout Fishing Association Home Page Its packed with all the info any visitor should ever need, and from may onwards is updated regularly.
One last bit of info that might be of help, is to buy Stan Headley's new trout fishing guide to orkney, between that and the otfa website, then you'll have more knowladge than most of the locals

Pick a copy up from w.s sinclairs in stromness. I believe its right up to date with flies etc and especially boat hire info. Give them a bell if you want one posted before you come up, to book ahead, no 01856851523.
God knows how long it took me to write all of the above, im a bit of a one finger wonder, when it comes to typing

So if i see you in may, you defo owe me a pint
Hope you have a good trip
Cheers Thorfinn