The key to fishing within the Ec is legacy targeting mode. Once that is activated in your options, you can use old-style-macros:
building macro:
- drag fishing pole in first macro slot
- choose wait-for-target in the next slot
- target- last target
(- maybe include a delay, if you want to repeat this macro)
You can even include this sequence multiple times, if you want to fish more often than 10-times with one keypress from a single spot.
From what I know, target-relativ is unique to UOA.
Not to forget, that you need to target the water manually before your macro works and after killing a sea serpent.
The EC's natural handle for your problem would be 'target-by-resource'. It works great for lumberjacking, so-so for mining and is not (yet?) implemented for fishing. On the other side you can build up, a 'one step forward'-macro easily in teh EC and repeat that one 8-times. (Thought you would have to target the water manually after each moving.)