Checking out the timing advance is a good idea. Do you have a NPO ? It should be rather easy to do then.
The interference on the sea may be coming from cells located 200km away. It might be coming from another country !
Check out the RXLEV and the RXQUAL distributions per TIMING ADVANCE. That will give you a good indication of the radio quality in your “outer” trx.
Or more simply, check out TRX indicators -> KPI on the outer TRX will perfecctly reflect the qos in the outer trx.
To overcome interferences:
either, use dummy neighbours to detect clean frequencies
or, apply a “wideband” SFH.