General Synonyms:
* Logical
* Consistent
* Clear
* Understandable
* Intelligible
* Rational
* Connected
* Cohesive
* Unified
More Specific Synonyms:
* Articulate (referring to speech or writing)
* Comprehensible (referring to something being easily understood)
* Well-organized (referring to a structure or argument)
* Sound (referring to a logical argument)
* Well-reasoned (referring to a logical argument)
The best synonym for "coherent" will depend on the specific situation.