You'll learn some of the stuff you'll need in your OW class, so don't stress out. Also, make sure that your instructor knows that you would like to dive from boats in the future so those skills can be practiced a bit more if you don't feel comfortable. You are paying for the instruction, if you need to practice something a bit more just speak up. Also, good instructors want to turn out SAFE divers so they are really interested in making sure you are comfortable with the skills they are teaching you.
If you still want practice after certification, talk to the local shops, captains, and divemasters, and explain your goal of going on the trips you've been looking at and ask if they can help you gain some experience/confidence (I have friends who've let us dive from their pontoons in lakes and reservoirs just for fun/practice). Chances are that either they'd love to help or they'll know someone who can.