In the Spanish armament of 1790, Seymour was called to service in command of the
ship of the line HMS Canada, opening his commission with a cruise off the Isle of Wight. Passing through shallow water, Seymour ordered the use of a
lead line to measure the depth ahead, but was accidentally struck in the head by the lead weight while soundings were being taken. Although little immediate damage seemed to have been caused, during the firing of a salute several days later Seymour suddenly suffered a severely adverse reaction and had to be taken ashore for emergency medical treatment. The head injury rendered him unable to endure any loud noises or bright lights and for the next three years he lived as an invalid at his country estate in
Hambledon. By 1793 Seymour was sufficiently recovered to return to service, and escorted
Lord Hood to the Mediterranean in
HMS Leviathan. There Hood led the occupation, defence and ultimate withdrawal from
Toulon during the
Republican siege of the city. Following the collapse of the city's defences, Seymour was sent back to England with dispatches but returned shortly afterward to convoy
Leviathan back to Britain.
Leviathan then engaged closely with
America, which she reduced to a battered wreck in a duel that lasted two hours.
Leviathan was also badly damaged, having taken fire from
Éole and
Trajan during the fighting. At Howe's order, Seymour then left
America (which was later captured) and joined the reformed fleet that held off a French counter-attack in the latter stages of the battle. In the aftermath of the action, Seymour was one of the captains marked out for praise, being presented with a medal commemorating his service during the engagement.
Leviathan had suffered 11 killed and 32 wounded in the engagement. In 1795, Seymour moved to the recently captured and soon became a
rear-admiral, engaging the French at the
Battle of Groix. Both ships were captured in heavy fighting, and
Sans Pareil suffered ten killed and two wounded during the exchange. In 1796, Seymour was employed in the search for the French fleet which attempted and failed to invade Ireland, but
Sans Pareil was badly damaged in a collision with
HMS Prince during the campaign and had to be decommissioned for extensive repairs. In April 1797, Seymour returned to sea with a small squadron of six ships searching the Eastern Atlantic for a Spanish treasure convoy. Although the convoy was eventually seized by a force sent by
Lord St. Vincent, Seymour had covered over 5,000 miles in his fruitless search. ==Admiralty service and death==