Everything you need to know about the SeaLegs marine weather API, marine forecasting, and getting started.
A marine weather API is a web service that provides weather forecast data specifically for ocean, coastal, and waterway conditions. Unlike general weather APIs that focus on temperature and precipitation, a marine weather API delivers data mariners and boaters actually need: wind speed and direction, wave height and period, swell data, visibility, and sea state classifications.
The SeaLegs API adds an AI analysis layer that translates raw data into plain-English summaries and vessel-specific condition ratings, so your users don't need a meteorology background to understand the forecast.
Wave height is the vertical distance from the trough to the crest of a wave, measured in feet or meters. Wave period is the time in seconds between successive wave crests passing a fixed point.
Period matters as much as height for comfort and safety on the water. A 4-foot sea with a 10-second period is a long, gentle roller that most boats handle easily. A 4-foot sea with a 4-second period is a steep, choppy mess that can be dangerous for smaller vessels. The SeaLegs API returns both metrics so your application can assess actual sea conditions, not just wave size.
Barometric pressure is the weight of the atmosphere measured in millibars (mb) or inches of mercury (inHg). For boaters and anglers, the pressure trend matters more than the absolute number.
Rapidly falling pressure often signals approaching storms and increasing winds. Steady high pressure (above 1020 mb) typically means calm, stable conditions. Many anglers also track pressure because falling barometric pressure often triggers feeding activity in fish. The SeaLegs API factors pressure trends into its overall condition analysis.
SeaLegs uses a multi-model approach that combines data from several numerical weather prediction models and applies AI analysis to produce more reliable forecasts than any single model alone.
The API includes confidence levels with each forecast so your application can communicate forecast reliability to users. Learn more in our post on understanding AI marine forecasts.
Wind direction describes where the wind is coming from, not where it's blowing to. "Wind from the east" (or an "east wind") means air is moving from east to west. A knot is one nautical mile per hour, roughly 1.15 mph.
For boaters, wind direction determines which shorelines and inlets are protected. A 15-knot east wind makes west-facing shores calm and east-facing shores rough. The SeaLegs API returns both wind speed and direction so your app can help users find protected water.
Most marine weather APIs return raw numerical data — wind speeds, wave heights, pressure values — and leave interpretation to the developer or end user. SeaLegs adds an AI analysis layer that translates raw data into actionable information:
This means your application can show users whether it's a good day to go boating without requiring them to interpret weather charts or understand meteorology.
Yes. The SpotCast API accepts a vessel_info parameter where you specify the vessel type (fishing, sailing, kayak, etc.) and length in feet. The AI analysis uses this to tailor the classification and summary.
A 4-foot sea rated "good" for a 40-foot sportfisher might be rated "poor" for a 17-foot skiff. This eliminates the need to build your own threshold logic for different boat types. See the SpotCast API deep dive for all available parameters.
The API provides global marine weather coverage. You can request a forecast for any latitude and longitude coordinates on water. Popular coverage areas include:
The AI analysis accounts for regional characteristics like local wind patterns, fetch distances, and coastal effects.
Each forecast from the SpotCast API includes:
See the API Reference for the full response schema.
Yes. The webhook system lets you submit forecast requests and get notified automatically when results are ready. You can attach your own metadata (trip IDs, customer emails, etc.) so webhook payloads arrive with all the context you need to trigger alerts in your application. See our webhooks tutorial for a full implementation walkthrough.
Yes. The SpotCast widget is an embeddable forecast you can add to any website with a single line of HTML. No API calls, no backend code — we handle everything. Widgets are configured in your developer dashboard and consume forecast-day credits based on their update frequency.
Yes, to get started. Every new developer account gets 10 free forecast-day credits with no credit card required. You can use these to test the full API, including SpotCast forecasts, webhooks, and widgets. After the free credits are used, you can purchase more on a pay-as-you-go basis. See our pricing page for details.
A forecast-day is one day of forecast data for one location. It's the billing unit for the API. If you request a 5-day forecast for one location, that uses 5 forecast-days. A 3-day forecast for two locations uses 6 forecast-days.
Every new account gets 10 free forecast-day credits. After that, credits start at $10 for 200 forecast-days. Credits don't expire.
Three steps:
Most developers have their first forecast running in under 5 minutes.
Developers use the SeaLegs API for a wide range of marine applications:
See 5 Ways to Use Marine Weather Data for more ideas.
Reach out to our team or dive into the documentation to learn more.