Not quite sure what you're asking here. If you're going to add a value
that's measured outside of your AWS system to the web page
presentation then it's going to have to be manually added at one point
or another. But there's no easy ** way AFAIK that a value can be
passed in from the outside and made known to Weatherlink. (But even if
this were possible it would still need to be a daily manual task to
pass in this value.)

Perhaps what you're doing now is to manually edit the web page and
then upload it again. Another option would be to manually edit the
..htx file on your local PC every day, having added a line to the
generic .htx file that shows the sea temperature somewhere on the page
and simply overtyping this text value every day with the latest value.
This would require some minimal knowledge of HTML but would be quite
easy to do.

And for anyone with a smattering of eg Visual Basic it should be
simple to write a little program that parsed the generic .htx file
looking for a placeholder string that you define like seatempxxx and
replaces it by a value that you enter via a small textbox. That way
you wouldn't even need to edit the .htx file manually each day.

There are of course other packages such as Weather Display that would
replace Weatherlink and that might (?) make it easier to add in
external values. But it's debatable whether it's worth changing your
whole software approach just to facilitate one small aspect.

** This isn't to say that there might not be clumsy workarounds like
somehow setting eg a leaf or soil temp value to the sea temp value in
eg the console and then using the relevant eg leaf temp HTML tag to
display the resulting value, but I can't immediately think how to do
this and it would almost certainly still involve some programming.
It's also worth bearing in mind that now that Davis have released
details of the Weatherlink API for add-on modules, you could in theory
write your own module (provided you speak C++) to pass in an external
value, though presumably still needing to mimic some existing
parameter value.

John Dann
www.weatherstations.co.uk