I am not a NA (Naval Architect) by any means so take what I say with a grain of salt
In my ancient (1967) Tartan 27' there are storage
compartments under the Vee berths on both the port and stbd sides where your drawing indicates you wan to 'make them open plan'. We have a water
tank forward under the Vee berth so no storage
locker there. Our openings on both sides are approx. 6" x 12" and have wooden (teak?) insert panels
to cover them.
There is limited storage space even on our slightly bigger boat
but what is nice is that there is access to it. Another option would be to cut a section of the horizontal Vee berth run that provided access to below with a removable section that simply pulls up and lies flat in it's cutout when the Vee berth cushions
are on top of it.
You did not say what year your Westerly was from but I have gathered that they are pretty stoutly built boats. If you were worried about reducing your boats structural integrity you could always add some extra members or 'sisters' to help with supplying rigidity.
Think about a load bearing wall that you want to add a doorway to. Now think about the same thing on a boat
that takes much more stress then your average house. In a house a carpenter
would add a 'header' to any load bearing wall to distribute the load to the sides of the new opening. This may mean attaching a structural framework to support your new cut away sections for your 'open plan'.
Frankly, I am not a big fan of open sections into the bilge
as you seem to want to do. Once the boat gets up on a heel everything will spill out from there so the horizontal access idea is much better. You still may want to add some support to your cut out section(s) that are horizontal and not vertical.
Your boat has 2 keels right?