Use a brand new 7/16ths bit- the harder the better. Ask the hardware store person what he suggests. (Don't ask the hardware store guy at a chain store unless he really seems to know what he is doing and is not simply filling in from the plumbing department
)
If you do it by hand just be sure that you lock down the casting securely so that you can control the drill only, not balance the drill in one hand and the casting in the other. You can clamp the casting to a table top and drill horizontally too- this is relatively easy since a table is easier to find than a solid vertical surface to clamp to. TLet the bit do the work, don't force it, keep it spinning from entry to removal. The casting is relatively soft metal and you are only opening the hole about an 1/8th (I think) so it is not that hard to do a good job- the bit will practically guide itself. As I often say- if I can do it, anyone can do it
Wait till sailing season is over too
Good luck