Welcome to the forums Domicrow! I hope you
lose your sanity have a good time here!
I'm afraid that crossing oceans is an incredibly difficult/boring/time consuming thing to do. You can't fast travel over them; you must manually swim through every tile. Don't get me wrong; it's possible but generally isn't worth it-though it does make a good challenge. If you still want to try here are a few tips:
1. Train swimming to a high level before attempting this. This will reduce the chance of you drowning.
2. Make sure that you do not try this in winter or in cold areas-the sea could freeze and kill you instantly.
3. Remember to take a supply of food and water with you! You won't find anything to eat in the middle of the sea (unless you meet a sea monster, which will probably kill you before you kill it) and you obviously can't drink seawater. Calculate well; carrying too much will slow you down and make you tired faster.
4. The easiest way to swim across an ocean is to be a vampire/thrall/husk, since none of them has the danger of drowning (they don't breathe). They also have the added bonus of not needing to eat, though a vampire actually requires fresh blood to survive, so that might be a problem. You can also try swimming across as a necromancer, which does not require food or water but can drown.
5. If the land form you are trying to reach is near arctic areas, you could try walking over the frozen sea. However this poses a big problem: a supposed bug can cause walls of ice to suddenly rise in the seabed, blocking your path and possibly killing you.
How to upload a picture (and add it to your comment):
1. Press the 'print screen' button on your keyboard. It's usually located next to the scroll lock button on the upper right section of your keyboard and is generally shortened to 'PrtScn' or 'Prt sc'.
2. Paste the picture into an editing program such as Microsoft Paint and save it.
3. Go to an image hosting site and upload the image. I personally like
imgur (which is free and doesn't require registering).
4. Once you've uploaded the image, click on it to open it in a separate page and copy that page's url. Then paste the url into your comment, highlight it and click on the 'image' button (right underneath the Bold button). This should enclose the image's url in img and /img.