Aluminium Bronze bodies with Monel Vanes are a common option for seawater applications. However, you can expect to pay three to fives times more for a monel valve over a stainless steel one.
The valve supplier should have a design criteria for the number of cycles. If you are requiring extensive cycles then advise the supplier and they should be able to change the design of the bellows to accomodate additional cycles. This could be by using an extended bellows and therefore only...