I get no response searching for Seamonkey in Synaptic.

Searching for SeaMonkey in Ubuntu Software Center I get a match.

In Terminal executing seamonkey, not SeaMonkey, all lower case, I get
The program 'seamonkey' is currently not installed. You can install it by
typing:
sudo apt-get install seamonkey

Sometime just entering a program on the command line in Terminal, Terminal
will tell you how to install it.