Six omnibuses in the Richard Mariner series by Peter Tonkin are $0.99/£0.99 at Kindle US/UK right now…
Ship to Shore, #1, contains The Leper Ship, The Bomb Ship, and Tiger Island
Buy at Kindle US | Kindle UK
Blue Blood, #2, contains Wolf Rock, Cape Farewell, and High Wind in Java
Buy at Kindle US | Kindle UK
Trouble at Sea, #3, contains The Fire Ship, Hell Gate, and Thunder Bay
Buy at Kindle US | Kindle UK
Sea of Troubles, #4, contains Power Down, The Coffin Ship, and The Pirate Ship
Buy at Kindle US | Kindle UK
In Too Deep, #5, contains Titan 10, River of Ghosts, and Meltdown
Buy at Kindle US | Kindle UK
Deep Blue Sea, #6, contains Iceberg, The Prison Ship, and Deadly Impact
Buy at Kindle US | Kindle UK
Almost all (maybe all) of the individual titles have been on sale before, and some of the omnibuses. And some have been re-issued, so prior purchases may not show up. So check carefully before you buy…