Fewer is better, remember you can always return to this tab and change your selections.

Select the input sources (3) you want to access on a regular basis. The Language and Text preference pane will open to the Input Sources tab. Check the option to Show Keyboard & Character Viewers in menu bar (1), then click on the Input Sources… button (2). Open System Preferences: Keyboard and select the Keyboard Pane. To make this function useable, you'll need to do a little setup. I know I can change my pass (and did) but (I probably shouldn't mention this) I know that I have a ~ somewhere in 1 or 2 of my other accounts' passwords ( not just email ) and I don't want to have to go through the hassle of getting to my Mac to change a password to access an account that I-probably-won't-need-to-but-needed-to-on-my-iPhone-at the-time-by-the-time-I-got-back-to-my-Mac.Although it isn't exactly what you asked for, one possible solution is to use the built-in ability to rapidly switch Input Sources (aka keyboard layouts). I also made a test case to confirm that a ~ on the Mac will work but typing it on the iPhone won't work.

I also have to clarify that I did make sure I was typing my password correctly. I changed one of my passwords last night to contain a '~' (which is now chaged again) but it seems that the ~ on the iPhone is a different Character to the '~' on the MacbookPro that you type by pressing 'Shift ` '. Seems obvious Press '123' then press '#+='. My question is how do I type the '~' (tilde?) icon on an iPhone. This isn't even an account I use for anything I do with my Apple products (iMessage etc) but I was unable to cancel the popup. I woke up this morning unable to use my phone because I had a constant message telling me to 'Type in the password for the ***Account.

How to make tilde symbol on keyboard