Is there a site that can split characters into radicals?
The online chinese dictionary MDBG provides radical information for every character in its database. For instance, if you search for the character 天 (tiān) and click on the first result, the "Rad/Str" column reads 大 + 1, i.e., the radical 大 plus one stroke.

Zhongwen.com also gives information on character decomposition. The entry for 洋 reads "Water 水 with 羊 phonetic." 

As Alenanno pointed out, not all parts of characters are radicals, but these two resources can give you more information about how characters are constructed.