I am aware of the elios and think their solution is excellent.
There is also group of companies looking into autonomous flight of UAVs within ballast and cargo tanks and I think the pre-mapping idea is the prefect solution to navigating around such tanks. http://www.bureauveritas.com/home/news/business-news/bureau-veritas-joins-the-recomms-drones-project
Personally I would like to see drones developed towards making life easier and safer for ship crews. As well as inspection of tanks, I think there could be systems created to aid recovery of people falling overboard, anti-piracy and looking long term, automated ships. A company called Sky-Futures https://www.sky-futures.com/ has entered the offshore market as well as Cyberhawk https://www.thecyberhawk.com/.
There is a lot of space in the maritime industry for UAV businesses to get started. Most people have no idea about the operational side to the maritime industry, most people I know think "cruise ship, container ship, fishing boat, oil tanker sinking, environmental disaster". I am happy to say there is way more involved, all that is needed is an increased awareness of the industry - that is where I can help.
To answer your question, when talking about structural inspections, the major concern is corrosion of the steel.
Ships crews will inspect tanks and vessel structures on a bi-annual basis, which is extremely dangerous and results in considerable loss of life every year. On offshore platforms usually rope access teams are used to literally climb down over the water to inspect and repair external structures.
There is a lot of international and national standards that are imposed on ships due to the potential loss of life, environmental impact and commercial risk of transporting goods by sea.
There are 3 main bodies who need to be satisfied that a ship is maintained to correct standard:
All three bodies have their own registered inspectors who are qualified marine professionals and can be found in almost any country around the world. These inspectors are one of the groups who will employ UAV services.
Here are some links your readers might find interesting.
Not UAV related, but interesting all the same