I need to replace my lower units and found a good set , but the cooling water pickups have been welded shut. My boat does get its cooling water from under the hull " high speed pickup " and the lower unit pickups are not used. My old lower units were just open but blocked off inside the boat. Should I pass on these? Or drill a drain hole at the lowest point or spray foam the cavity and seal them up. My boat is in the water all year and is pulled in the spring for drive service and goes right back in. Help!

Bravo drives, depend on water flow through them for cooling. Typically if the water is not needed to cool the engine, then it is allowed to dump out the transom or back on to the drive to further help cooling. But you should have water flowing through the pick ups and the entire drive to cool it. The new lowers could possibly be drilled out to get that capability back, otherwise I would pass on them.
Hope that helps.
Dick

These drives have showers , does that change your thoughts?

No, the amount of cooling by showering water on the outside is much less, in my opinion, to the water running through the drive. Both at the same time is advised. You could go to an external cooler setup with a pump and filter. I have converted a few to that type of system..