On both ferry journeys you have the opportunity to go out onto the deck, and this is certainly something we would recommend for the awesome views! However, please be aware that, regardless of the time of year, it will be cold and windy on deck. Consequently, we would strongly recommend bringing/wearing layers (e.g. t-shirt, fleece, warm jacket), a hat, gloves and a waterproof jacket (waterproof trousers can also come in useful if you have them!).
On Orkney itself there is some walking involved and most of the sights are outdoors, with Ring of Brodgar in particular being in quite an exposed location, where the ground can be muddy and slippery. We would definitely recommend that you wear hiking boots or, as an absolute minimum, sensible shoes/trainers/sneakers which have good grip and which are suited to this type of terrain.
Even if the sun is shining brightly when you leave Inverness, they do say that you only really experience Scotland when you see four seasons in one day, so please be prepared – it’s better to leave extra layers on the coach unused than to feel cold and wet on such a long day!