Although visibility in the water is not as good in the afternoon due to the position of the sun, late afternoon appeared to be the time when large schools of fish would having a feeding frenzy in the waters alongside Yulia's.
Because visibility in the afternoon is so good, it is well worth the effort getting across to Pulau Rubiah to snorkelling in the relatively calm waters of the west side. You will find a wide variety of small, colorful fish scavenging amongst the broken coral in depths of only 1 to 2 metres.
The west side of Pulau Rubiah is were the boats drop passengers travelling from Iboih beach. The water is much calmer on this side and visibilty in the afternoon is very good. It can get quite crowded on weekends. But if you are into clams, this is the place to go. There are a huge number of vividly colored clams that are thriving in the warm shallows - from iridescent blue to bright green, the clams seem to have taken over when the coral died away.
On the east side of Pulau Rubiah are the 'sea gardens'. Snorkellers can access them from a small beach - beware the changing tide if you leave things on the beach. The water can be fairly choppy and there are many rocks to navigate before getting into deeper water. In the shallower parts, the coral was disappointing - like most of the shallow water coral around Pulau Weh, it had suffered from the rising water temperature. Still there were plenty of fish to see.
Stronger swimmers can navigate around a point heading north where there is the chance to see larger fish and eels. There are good sandy bottom dive spots for novice divers in this area and visibility in the middle of the day is generally good.
It is possible to swim to Pulau Rubiah - about 200 metres from Iboih depending upon your starting point. The currents can be strong so it is recommended to use fins/flippers and probably best not to swim alone. It is not difficult to take a sarong, a camera, and a small amount of money in a waterproof bag. There is a small restaurant on Rubiah selling drinks and biscuits.
The alternative is to take a boat. On busy weekends and holiday periods, locals from Banda Aceh or further away will spend a few days on the island. There are frequent boats - I think the cost was 25,000 rupiah per person - to Rubiah. These boats also do short trips arond the island.
If there are no 'public' boats, your accommodation should be able to arrange transport, but is is likely to be more expensive.
It is certainly worth an afternoon visit for short walks and snorkelling.