I would like a lens that I can shoot wide angle and macro.
Everybody would.... your 12-50mm lens is about the most versatile lens available for any type camera to fit that request.
To get wide angle you need a dome port. Unfortunately dome ports usually are not a good choice for macro.
The Olympus 12-40mm f/2.8 lens is a very nice lens above water. Its minimum focus distance is 20cm (8 inches) from the sensor. Because of this it is not recommended for underwater use. It will not make you as happy as your 12-50mm. You probably would not be happy at all trying to use the 12-40mm for wide angle underwater and you would spend quite a bit more than the 12-50 anyway.
Macro
Because of the minimum focus distance for the 12-40mm lens it can only go to 0.3 x magnification (meaning a 56mm wide subject would fill the image) and adding a wet mate close-up lens is still not going to get you to 1:1. The 12-50mm will do much better.
The 12-50mm lens with it's macro mode (sets lens to 43mm focal length) is extremely popular. Are you using an Olympus housing, a Nauticam, or what?
If you have Nauticam, they make a port & gear set that allows you to switch to macro mode and regular zoom mode while underwater. The port and lens gear set is not cheap, but is less expensive than buying a macro lens & port plus a medium wide lens & port.
The Olympus 60mm macro lens is well regarded, people like it.
The 12-50mm lens in macro mode can make a subject 33.5mm wide fill up the image (that's not true 1:1, but quite close and we did 'real world testing' not a laboratory test.)
The 12-50mm lens in macro mode with the Nauticam SMC will let you fill the image with a subject 25mm wide (one inch) if you can get that close to the creature.
The 60mm lens, in real world testing, got true 1:1 reproduction, a creature 17mm wide will fill the image.
The Olympus 60mm lens with the Nauticam SMC will photograph much smaller subjects... as little as 11mm wide will fill the image. That's a little more than 1/2 the diameter of a U.S. penny.
And trying to find creatures that small, and then trying to find them in the viewfinder, is not easy, takes practice and patience.
