ISTA Procedures, and other industry-standard packaging test protocols, don’t contain vibration (PSD) profiles for ocean shipment.  Several studies have shown that vertical vibration during sea transport is so small that it’s negligible. This is not to say that ocean shipment is benign – there are shocks due to loading and unloading, ship motions can cause the shifting of goods, there is potential damage from extreme temperatures and humidity, and compression forces due to stacking can be large.  Since essentially every ocean shipment also involves other kinds of transport (road, rail, air) to and from the port, it has been fairly standard industry practice to simulate those other transport vibrations with the assumption that they envelope any vertical vibration experienced during ocean transport.  Regarding shocks related to ocean shipment:  there is not a lot of data for loaded sea containers, but what information we have suggests that the shocks are fairly low.  The primary shock hazard to ocean-freight packages is probably when they are loaded into the container and unloaded from the container, and are handled at other points during distribution.  

The standard practice we see utilized at this time is to select an appropriate test as if the packaged-product were going to be surface-transported only.  If you think dockside loading/unloading might be more severe, you could increase the impact velocities and drop heights.  You could also add a flat drop or two (to simulate handling with cargo nets), and perhaps some rotational corner drops as well.  For stacking, to better simulate the possibly more severe conditions, consider increasing the "F" (compensating) factor above the required value.  I'd recommend going to one or more of the atmospheric conditioning extremes (or beyond).  You can also consider adding actual water spray to dampen the packages.  

Utilizing ISTA 4AB to establish a test sequence which includes these additional handling events to your established protocol is another potential path.  The ocean freight leg of the journey typically isn’t creating catastrophic failures that are preventable with packaging.  However, I do think that the cumulative effects are contributing to the fatigue of a package which then when on land, potential yields failures at a quicker/higher rate.  More to come from ISTA on this subject, till then I would love to hear more about your challenges and success with evaluating these hazards against your packaged-products.  The real world correlation piece from a variety of product manufactures is always the most useful information in the development & validation of a potential test procedure.  

Below are a couple documents talking about the low vertical vibration levels while on a ship. 

• Measurement and Analysis of Vibration and Temperature Levels in Global Intermodal Container Shipments on Truck, Rail, and Ship” (Singh et al. 2012)
  
• “This paper reviews various previous studies conducted on measurement of physical and climatic conditions that occur inside International Standard Organization intermodal containers and their impact on packaging. The study measured vibration and temperature levels in two different shipments from India to USA where recorders measured the acceleration levels and temperature data while the instrumented and loaded containers travelled on truck, rail and ship. Measured data shows that extreme vibration levels occur while containers travel on trucks on poor road conditions followed by rail and ship travel. Highest temperature levels occur inside these containers when they are stationary and are at dry docks, as opposed to when they are travelling on truck or rail. Lowest vibration levels and temperature extremes occur when they are loaded on a ship and are travelling on sea.” 
  

• Measuring the package shipping environment in refrigerated ocean vessels. Singh, S.P., G.J. Burgess, J.A. Marcondes, & J.R. Antle (1993) Packaging Technology and Science, 6(4), 175-181.
  
• “In summary, the results from these previous studies show that vibration levels are different among different modes of transportation, and are generally higher in truck and rail transport than ship transport.”
  
• “The vibration and shock levels were generally very low when the package system was being transported by ship.”
  
• “The vertical vibration produced the highest overall Grms value of 0.209g and represented truck shipments. Data is presented for all three orientations - lateral, longitudinal and vertical. Highest results from the vessel or ship were at 0.127g clearly showing very low level of movement as compared to truck transport in both continents.”