If you are managing a dive somewhere, the plans below can be of use:
Dive Planning Software
Subsurface – This is an open source dive log which can download your dives off the dive computer, lets you input additional details, and plan future dives.
MacDive – If you run a Mac computer, MacDive is an option to log your dives and backup your dive computer profiles.
Pastoblend – Useful if you have to carry out any gas blending. Supports Nitrox and Trimix.
Dive Planning Websites
EasyTide – Tide Prediction
Weather Forecast – Yachting & Boating World Weather