Therefore, You can enjoy MW DXing much better at a sea cliff or a sea coast than that in inland area.
The sea gain was found long time ago by P.Knight, a researcher of BBC around 1975 and the sea gain was defined with some formulas in the ITU-R recommendation P.1147-4 (Click)
" Prediction of sky-wave field strength at frequencies between about 150 and 1700 kHz". The sea gain depends on the distance between transmitting point and receiving point and frequencies. The graph below is an extract from the ITU-R recommendation. The curve A is for MW band. For MW DXing, the propagation distance would be more than 6000 km then the sea gain is expected about 10 dB. This means your S-meter goes up about 3 scales(I mean when your S-meter indicates S-1 at a inland area, you can expect S-4 signal at a cliff or a coast.) You should not ignore this effect for much better MW DXing.
Some papers written by P.Knight et al of BBC are downloadable. The clickable links are as follows.
There are very few articles of MW DXing that report about the sea gain. But we can read a very interesting article titled " Inland versus seaside MW DXing".(Click)
written by John Plimmer from South Africa who is one of famous MW DX information web site DXing info Community. ((http://www.dxing.info/)
The following video shows some reception scenes of KKOL (Seattle) on 1300 kHz at a cliff in Chiba, Japan on Sep.22 2016. The audio clips and pictures are provided for my blog by one of MW Japanese DXers, Sawapon (Sawapon is his nickname.) Sawapon's blog(Japanese) is here (click).. We conducted a DXpedition with other MW DXers on the same day. The signal of KKOL propagating 7700km from Seattle was received well even with a portable MW radio thanks to the sea gain.