Seattle has invested more into its receivers than any other position on the roster whether you're talking about the big deals given to Sidney Rice and Percy Harvin or second-round picks used on Golden Tate and Paul Richardson. But losses due to free agency (Tate), injuries (Rice and Richardson) and attitude (Harvin) leaves receiver as one of a few needs on this team, even with the addition of Jimmy Graham.
However with investing so heavily into the receivers the philosophy also seems not to put a huge value on 1st round receivers. Whether you look at where John Schneider comes from in GB where their most productive receivers have typically been 2nd or 3rd round guys (Greg Jennings 2nd rd, James Jones 3rd rd, Jorday Nelson 2nd rd, Randall Cobb 2nd rd) or the more recent history in Seattle (Paul Richardson 2nd rd, Kevin Norwood 4th rd, Chris Harper 4th rd, Kris Durham 4th rd, Golden Tate 2nd rd.) So I think the OP's thought of looking at later round WR's is right on spot.
Some guys that I will be keeping an eye on...
Jaelen Strong - ASU- 2nd round - There is just something about Jaelen Strong that makes me see him as a big time playmaker, something Pete and John covet. He's not the "freakish" size speed prospect of a DGB or some others but he's damn good and just seems to have a knack for the big play. I see him as a comparison to a pre-injury Michael Crabtree.
Phillip Dorsett - Miami - 2nd - Your classic small (5'10") speedster (4.3 40), Dorsett would fill the speed threat in case P-Rich isn't ready day 1 plus he would give us an option for returning kicks/punts.
Dezmin Lewis - UCA - 4th/5th - Good size (6'4" 214) speed (4.5 40) combination who really opened a lot of eyes at the Senior Bowl. Scouts have said he's got "top notch" body control and that he moves very smoothly for a guy his size. Scouts also like his run after the catch ability making him more than just a possession receiver.
Darren Waller - Georgia Tech - 5th/6th - Waller is a huge receiver standing 6'6" and weighing in at 238 Lbs. I don't know if he can play WR in the NFL but it's not due to a lack of speed (he ran a 4.46 40 @ the combine) but he's stiff and doesn't have the quick twitch movement that the elite NFL WR's have. Still as a one trick pony and redzone threat he can be an asset. Waller may also be a candidate to move to TE if he can learning to block at the line of scrimmage and not just in the open field.
DeAndre Smelter - Georgia Tech - 6th/7th - Smelter is an intriguing prospect, once thought of as a 1st or 2nd round guy he's kind of fallen off the map after tearing his ACL. He's got good size at 6'2" and 226 Lbs. and even though we don't know his 40 he was obviously faster/quicker than his teammate Darren Waller on film so it's probably safe to say he's at least a 4.4's guy. He's got giant hands measuring at a whopping 11 inches (Bigger than Russell, Gronk, Calvin, Marshall, Dez, Demaryius, etc). He also comes from a running team with an athletic QB that scrambled around frequently like Russ so he knows how to block and how to continue working to get open while his QB is outside the pocket.
Kasen Williams - Washington - UDFA - Pete's brought in a few local guys recently, specifically Husky WR's (Kearse and Kevin Smith). I can see him eying another local product in Williams that should fly under the radar because of the troubles he had with injuries and poor QB play at Washington. Kasen Williams was the #2 ranked WR in the country coming out of HS and he brings a good combination of size (6'3" 217) and athleticism (4.5 40, 35.5 vert) if he can regain his form after recovering from a broken Fibula and Lisfranc injury. He did not look good at Washington last year but I expect Pete to kick the tires and see if the talent he once possessed is still there in some form.
Kyle Prater - Northwestern - UDFA - Prater was a Pete Carroll recruit that committed to USC but didn't play due to season ending injury his first year. He then transferred to Northwestern after Pete had left and Lane Kiffin took over. He didn't play much his first year at Northwestern due to learning a completely different style of offense and coming off his injured foot. His senior year he took over as the #1 starter and lead the receiver group in catches, yards, and touchdowns. He's got great size at 6'5" and 228 Lbs. He's good at making the contested catch, isn't afraid to take a hit going over the middle, and he's very smooth and fluid in his movement. I didn't see him create much separation in running his routes but he used his size and his body well to position for the ball when it got there. Don't be surprised if Pete reaches out to "recruit" Prater yet again.