OK, I do know the Sea isn’t a tree! It is one of the Ogham symbols, however – Mor. 
Mor is one of the “extra” Fedha (trees) of the Ogham system. They are are called Forfedha. The five symbols for the Sacred Grove-Koad; Spindle-Oir; Honeysuckle-Uilleand; Beech-Phagos and Sea-Mor were added to the original twenty at a later date. To me, this supports the Druids’ belief that fluidity and change were essential to inherent wisdom.
In Celtic Earth Magic Mor is one of the six Master symbols. It is awakened in the energy field of a student when they have progressed through and completed the first three levels, and are ready to surrender to the world of Earth Magic, true enlightenment and the secrets of the Spiritual aspects of the rest of Nature.
The Sea is said to hold hidden secrets yet to be discovered. All bodies of water connect to our subconscious and unconscious. There are depths of the oceans which cannot be reached, just as there are depths within us which cannot be analysed and explained. These are the mysteries which align to our Spiritual selves – the part of us which is Divine.
If we sit quietly by the sea, close our eyes and align our breathing to the ebb and flow of the tide we may begin to get a sense of these hidden depths within ourselves. The more we expand our awareness and step outside of the “thinking” box the closer we get to flowing in tune with the rhythm of the rest of Nature. This is when she may open herself to us, and trust us to respect and honour her secrets.
When a student receives this beautiful symbol into their energy field the mysteries of the ocean begin to sparkle in their consciousness like sunlight on water.