Well if your older worlds were created before TU31 and you explored your entire map, then no you will not get Ocean Monuments in those older worlds.
As for the "newer" worlds created after TU31 update, then you need to look in the "Deep Ocean" biomes, if you have created 6 so far and haven't found one yet, try looking in the seeds section of either the PS3 forums here, or try also the Xbox360 forum section for seeds there. There have been quite a number of seeds posted with people finding Ocean Monuments and they'll even in some cases list the co-ordinates of where to find them.
Also you can go to YouTube and search TU31 console seeds, you'll find loads of people posting videos, with quite a number of them containing Ocean Monuments, because being one of the latest and coolest additions to the update, loads of people want to show off!
If you have a current world. New updates are possible to that world if you haven't spent more then 3 minutes in parts of that world. But if you take same seed and restart it from beginning it will switch over to New update. It can possible add temple. Or not. But I have 3 world I love on 360 and they are 306959825. -303721893827871 and billysett92 all have ocean Monuments. -303721893827871 has all 3 temples/monuments and cool biomes and lots of land. My ultimate favorite
I was seed hunting for about 5 days on the ps3. I would load a new map in creative and roam it, only to find it was not suitable. I could get maybe 5 to 10 maps searched in a day. Then I started just using the website MineAtlas. I would look at hundreds of maps in 10 minutes, and if any looked tempting I would load it in creative, and roam about to see if it was what I wanted. After another 5 days, and many loaded maps. I came across the seed I am now using, and it has everything I personally wanted. OM, Messa, Ice Spikes, and many more. It does not have a jungle biome, but i am currently looking for some seeds in chests... no luck as of yet