Info on the upgrade certs can be found on Marriott's website: Room Upgrade Information

Basically, each 5K cert will give you a one category upgrade for one night. It looks like for your situation, an Ocean Front room is considered 3 upgrade categories from a base standard room.

One nice benefit of Marriott's program is that you can make a hotel reward reservation and not have the needed points at the time of reservation. You will need to have the points in your account before the actual stay, but you can make the reservation now.

As for whether guaranteeing an Ocean Front room is a good use of points or not is somewhat of a personal nature. I have to say that I've been pretty lucky and most of my beach resort stays have been upgraded to Ocean Front rooms, but your Plat upgrade isn't guaranteed. If the hotel gets booked up and doesn't have any left when you check-in, then you would be out of luck. So the question is, how important is that upgrade? If not getting it would be a major impact on the enjoyment of your stay, then I would suggest taking no chances. Reserve the Ocean Front and pay points for it.

If getting the Ocean Front room won't be a big issue, then take your chances.

With that said, what I might suggest, if the upgrade is really important, is to reserve the upgraded room and when you check-in, ask if the hotel would be willing to give you Ocean Front as an upgrade (complementary) and let you return the upgrade awards back to Marriott. It may work, it may not. But you will still have your upgraded room either way.