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
Kindle US:
https://www.amazon.com/dp/B07DFNSRSR
Kindle UK:
https://www.amazon.co.uk/dp/B07DFNSRSR
Blue Blood, #2, contains
Wolf Rock,
Cape Farewell, and
High Wind in Java
Kindle US:
https://www.amazon.com/dp/B082FC6GYC
Kindle UK:
https://www.amazon.co.uk/dp/B082FC6GYC
Trouble at Sea, #3, contains
The Fire Ship,
Hell Gate, and
Thunder Bay
Kindle US:
https://www.amazon.com/dp/B0B71W81XM
Kindle UK:
https://www.amazon.co.uk/dp/B0B71W81XM
Sea of Troubles, #4, contains
Power Down,
The Coffin Ship, and
The Pirate Ship
Kindle US:
https://www.amazon.com/dp/B07K9Z1V92
Kindle UK:
https://www.amazon.co.uk/dp/B07K9Z1V92
In Too Deep, #5, contains
Titan 10, River of Ghosts, and
Meltdown
Kindle US:
https://www.amazon.com/dp/B0B7TBGMND
Kindle UK:
https://www.amazon.co.uk/dp/B0B7TBGMND
Deep Blue Sea, #6, contains
Iceberg,
The Prison Ship, and
Deadly Impact
Kindle US:
https://www.amazon.com/dp/B0BB36SYD2
Kindle UK:
https://www.amazon.co.uk/dp/B0BB36SYD2
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...