I think your question is a bit of a conumdrum. How can I diversify without being different?
If you have enough budget, you might consider a Blancpain Fifty Fathoms or an IWC Aquatimer. Either of these lines of watches would give you the water integrity you seem to demand without duplicating the look of the Seamaster you currently own.
Or you might consider an Omega Seamaster Bond Commander Limited Edition. This doesn't seem to be a very popular watch around these parts, but it would allow you stick with the brand you're familiar with, Omega, and give you a different look with its white dial and markers. Many seem to put off by the "007" as the counter-balance on the end of the sweep second hand.
That's about all I can come up with to help.
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