So here's the first post on how I plan a sea fishing trip. We'll start with the online tools that I use to research my trips. At this point it's worth noting, I don't fish where I want to fish as such, I fish where I have the best chance. That is dependant on conditions, tides, weather, seasonality etc. In my mind this is what makes a good fisherman, knowing how to be in the right place at the right time.
The first tool I use that helps you do this is a tide times website. There are various ones to use, but I use one called tides4fishing, as it shows you moon phases (don't get too hung up on them, just know that a new moon has the biggest tides in a lunar month, and a full moon has the second biggest tides in the month, in between that are the small tides, or neaps) and also the tidal coefficient (this number is a value given to how big a tidal range is. The bigger the number, the more tidal movement you will see).
Many venues will fish well at different states of tide. Some places will fish best 3 hours either side of low water, some a couple of hours of low water, some will just fish well on the outgoing tide and so on. We'll get into that later though. But this will help you plan where to fish with your window of time that you have available.