I would strongly advise against throwing a tennis ball. The lighter weight is potentially more stressful on the arm. It’s also just completely different – no seams, smaller size, etc Seriously, you’re better off doing nothing in my opinion if a tennis ball is your only option.
Can’t you just play catch in the rain into a net or with a throwing partner? I mean, soccer players play in the rain. Football players play in the rain. Lacrosse players play in the rain. Wear sleeves. Grab an old baseball and/or glove that you don’t mind getting wet, and get out there. No excuses. Come on, man.